CRISPR/Cas9-mediated reporter knock-in in mouse haploid embryonic stem cells

A Simple and Efficient Procedure for Developing Mouse Germline Stem Cell Lines with Gene Knock-in via CRISPR/Cas9 Technology

Cultured mammalian spermatogonial stem cells (SSCs), also known as germline stem cells (GSCs), hold great promise for applications such as fertility preservation, gene therapy, and animal breeding, particularly in conjunction with accurate gene editing. Although the in vitro development of mouse GSC (mGSC) lines, and gene-targeting procedures for such lines, were initially established about two decades ago, it remains challenging for beginners to efficiently accomplish these tasks, partly because mGSCs proliferate more slowly and are more resistant to lipid-mediated gene transfection than pluripotent stem cells (PSCs). Meanwhile, methods for mGSC culture and gene editing have been evolving constantly to become simpler and more efficient. Here, we describe how to develop mGSC lines from small mouse testis samples and how to carry out gene knock-in in these cells using CRISPR/Cas9 technology, detailing three basic protocols that constitute a streamlined procedure. Using these simple and efficient procedures, site-specific knock-in mGSC lines can be obtained in 3 months. We hope that these protocols will help researchers use genetically modified GSCs to explore scientific questions of interest and to accumulate experience for application to GSC research in other mammalian species. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Establishment of mouse GSCs lines from small testicular samples Basic Protocol 2: Preparation of plasmids for gene knock-in using the CRISPR/Cas9 system Basic Protocol 3: Establishment of gene knock-in mGSC lines by electroporation gene delivery.

Efficient tagging of endogenous proteins in human cell lines for structural studies by single-particle cryo-EM

CRISPR/Cas9-based genome engineering has revolutionized our ability to manipulate biological systems, particularly in higher organisms. Here, we designed a set of homology-directed repair donor templates that enable efficient tagging of endogenous proteins with affinity tags by transient transfection and selection of genome-edited cells in various human cell lines. Combined with technological advancements in single-particle cryogenic electron microscopy, this strategy allows efficient structural studies of endogenous proteins captured in their native cellular environment and during different cellular processes. We demonstrated this strategy by tagging six different human proteins in both HEK293T and Jurkat cells. Moreover, analysis of endogenous glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in HEK293T cells allowed us to follow its behavior spatially and temporally in response to prolonged oxidative stress, correlating the increased number of oxidation-induced inactive catalytic sites in GAPDH with its translocation from cytosol to nucleus.

Editing SOX Genes by CRISPR-Cas: Current Insights and Future Perspectives

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and its associated proteins (Cas) is an adaptive immune system in archaea and most bacteria. By repurposing these systems for use in eukaryote cells, a substantial revolution has arisen in the genome engineering field. In recent years, CRISPR-Cas technology was rapidly developed and different types of DNA or RNA sequence editors, gene activator or repressor, and epigenome modulators established. The versatility and feasibility of CRISPR-Cas technology has introduced this system as the most suitable tool for discovering and studying the mechanism of specific genes and also for generating appropriate cell and animal models. SOX genes play crucial roles in development processes and stemness. To elucidate the exact roles of SOX factors and their partners in tissue hemostasis and cell regeneration, generating appropriate in vitro and in vivo models is crucial. In line with these premises, CRISPR-Cas technology is a promising tool for studying different family members of SOX transcription factors. In this review, we aim to highlight the importance of CRISPR-Cas and summarize the applications of this novel, promising technology in studying and decoding the function of different members of the SOX gene family.

Gene insertion in Saccharomyces cerevisiae using the CRISPR/Cas9 system

The aim of this work was to rapidly and efficiently insert target DNA sequences into predetermined genomic sites in Saccharomyces cerevisiae. In this study, we designed two technical routes for gene insertion in the S. cerevisiae genome based on the CRISPR/Cas9 system, and a CRISPR array was inserted into the Amp site and the crRNA site of the pCRCT plasmid, respectively. The CRISPR array consists of a 100 bp donor sequence, the target gene and guide sequence. A 100 bp donor sequence was designed to have two 50 bp homology arms flanking the Cas9 cutting site and incorporate 8 bp or 1000 bp deletions including the PAM sequence, where the target gene was also inserted. The results showed that using only one pCRCTG plasmid and a 100 bp dsDNA mutagenizing homologous recombination donor, we can successfully insert a 2.9 kb gene fragment at the target site of the S. cerevisiae genome. However, inserting the CRISPR array into the crRNA site has a higher recombination efficiency than inserting into the Amp site. This recombination strategy represents a powerful tool for creating yeast strains with target gene inserts.

Zfp281 Shapes the Transcriptome of Trophoblast Stem Cells and Is Essential for Placental Development

Placental development is a key event in mammalian reproduction and embryogenesis. However, the molecular basis underlying placental development is not fully understood. Here, we conduct a forward genetic screen to identify regulators for extraembryonic development and identify Zfp281 as a key factor. Zfp281 overexpression in mouse embryonic stem cells facilitates the induction of trophoblast stem-like cells. Zfp281 is preferentially expressed in the undifferentiated trophoblast stem cell population in an FGF-dependent manner, and disruption of Zfp281 in mice causes severe defects in early placental development. Consistently, Zfp281-depleted trophoblast stem cells exhibit defects in maintaining the transcriptome and differentiation capacity. Mechanistically, Zfp281 interacts with MLL or COMPASS subunits and occupies the promoters of its target genes. Importantly, ZNF281, the human ortholog of this factor, is required to stabilize the undifferentiated status of human trophoblast stem cells. Thus, we identify Zfp281 as a conserved factor for the maintenance of trophoblast stem cell plasticity.

Haploid pluripotent stem cells: twofold benefits with half the effort in genetic screening and reproduction

Haploid pluripotent stem cells, which are capable of self-renewal and differentiation into other cell types with only one set of chromosomes, have been established in several species from haploid embryos. Compared with diploid embryonic stem cells (ESCs), haploid embryonic stem cells (haESCs) are smaller in size, have a prolonged metaphase, and undergo self-doubling during culture. The monoallelic expression of haESCs provides great convenience for recessive inheritance research. Genetically modified haESCs also provide benefits in replacement of the gamete genomes, which not only facilitates the study of the function of imprinted genes but also potentially removes barriers to same-sex reproduction. In this review, we focus on strategies for obtaining haESCs and their potential applications in genetic screening, genomic imprinting, and unisexual reproduction.

Efficient SNP editing in haploid human pluripotent stem cells

PURPOSE

To correct a potentially damaging mutation in haploid human embryonic stem cells.

METHODS

Exome sequencing was performed on DNA extracted from parthenogenetically derived embryonic stem cell line (pES12). An SLC10A2 gene mutation, which affects bile acid transport, was chosen as mutation of interest in this proof of concept study to attempt correction in human pluripotent haploid cells. Confirmation of the mutation was verified, and guide RNA and a correction template was designed in preparation of performing CRISPR. Haploid cells underwent serial fluorescence activated cell sorting (FACS) with Hoechst 33342 to create an increasingly haploid (1n) enriched culture. Nucleofection was performed on p. 37 and then cells were sorted for 1n DNA content with +GFP to identify the haploid cells that expressed Cas9 tagged with GFP.

RESULTS

104,686 haploid GFP + cells were collected. Cells were cultured, individual colonies picked, and 48 clones were sent for Sanger sequencing. CRIPSR efficiency was 77.1%, with 7/48 (14.6%) clones resulting in a corrected SLC10A2 mutation. Confirmation of persistence of haploid cells was achieved with repeated FACS sorting and centromere quantification. Given the large number of passages and exposure to CRISPR, we also performed analysis of karyotypes and of off-target effects. Cells evaluated were karyotypically normal and there was no evident off target effects.

CONCLUSIONS

CRISPR/Cas9 can be effectively utilized to edit mutations in haploid human embryonic stem cells. Establishment and maintenance of a haploid cell culture provides a novel way to utilize CRISPR/Cas9 in gene editing, particularly in the study of recessive alleles.

Human Genomic Safe Harbors and the Suicide Gene-Based Safeguard System for iPSC-Based Cell Therapy

The use of human induced pluripotent stem cells (hiPSCs) and recent advances in cell engineering have opened new prospects for cell‐based therapy. However, there are concerns that must be addressed prior to their broad clinical applications and a major concern is tumorigenicity. Suicide gene approaches could eliminate wayward tumor‐initiating cells even after cell transplantation, but their efficacy remains controversial. Another concern is the safety of genome editing. Our knowledge of human genomic safe harbors (GSHs) is still insufficient, making it difficult to predict the influence of gene integration on nearby genes. Here, we showed the topological architecture of human GSH candidates, AAVS1, CCR5, human ROSA26, and an extragenic GSH locus on chromosome 1 (Chr1‐eGSH). Chr1‐eGSH permitted robust transgene expression, but a 2 Mb‐distant gene within the same topologically associated domain showed aberrant expression. Although knockin iPSCs carrying the suicide gene, herpes simplex virus thymidine kinase (HSV‐TK), were sufficiently sensitive to ganciclovir in vitro, the resulting teratomas showed varying degrees of resistance to the drug in vivo. Our findings suggest that the Chr1‐eGSH is not suitable for therapeutic gene integration and highlight that topological analysis could facilitate exploration of human GSHs for regenerative medicine applications. Our data indicate that the HSV‐TK/ganciclovir suicide gene approach alone may be not an adequate safeguard against the risk of teratoma, and suggest that the combination of several distinct approaches could reduce the risks associated with cell therapy. stem cells translational medicine2019;8:627&638

Derivation of Haploid Neural Stem Cell Lines by Selection for a Pax6-GFP Reporter

Haploid cells facilitate genetic screening of recessive mutations for a single set of chromosomes. Haploid embryonic stem cells (haESCs) have been achieved in several species and widely utilized in genetic screens. The fact that haESCs undergo substantial diploidization during differentiation has limited the screening to other haploid cell types. In this study, we report a method to establish haploid neural stem cells (haNSCs) by selection for a Pax6 reporter. We inserted a green fluorescence protein (GFP) marker gene by homologous recombination into the Pax6 locus of an haESC line. GFP-positive haploid cells could be sorted and further cultured in the NSC medium for more than 30 passages. The established haNSCs expressed neural lineage markers and could differentiate into neurons, oligodendroglia, and astrocytes. Our study shows the feasibility of deriving haploid proliferative somatic cell lines using a genetically encoded reporter that suggest a system for genetic screening of neural and retinal development.

A review of genetic engineering biotechnologies for enhanced chronic wound healing

Traditional methods for addressing chronic wounds focus on correcting dysfunction by controlling extracellular elements. This review highlights technologies that take a different approach - enhancing chronic wound healing by genetic modification to wound beds. Featured cutaneous transduction/transfection methods include viral modalities (ie adenoviruses, adeno-associated viruses, retroviruses and lentiviruses) and conventional non-viral modalities (ie naked DNA injections, microseeding, liposomal reagents, particle bombardment and electroporation). Also explored are emerging technologies, focusing on the exciting capabilities of wound diagnostics such as pyrosequencing as well as site-specific nuclease editing tools such as CRISPR-Cas9 used to both transiently and permanently genetically modify resident wound bed cells. Additionally, new non-viral transfection methods (ie conjugated nanoparticles, multi-electrode arrays, and microfabricated needles and nanowires) are discussed that can potentially facilitate more efficient and safe transgene delivery to skin but also represent significant advances broadly to tissue regeneration research.

Unbiased compound-protein interface mapping and prediction of chemoresistance loci through forward genetics in haploid stem cells

Forward genetic screens in haploid mammalian cells have recently emerged as powerful tools for the discovery and investigation of recessive traits. Use of the haploid system provides unique genetic tractability and resolution. Upon positive selection, these screens typically employ analysis of loss-of-function (LOF) alleles and are thus limited to non-essential genes. Many relevant compounds, including anti-cancer therapeutics, however, target essential genes, precluding positive selection of LOF alleles. Here, we asked whether the use of random and saturating chemical mutagenesis might enable screens that identify essential biological targets of toxic compounds. We compare and contrast chemical mutagenesis with insertional mutagenesis.

Selecting mutagenized cells with thapsigargin, an inhibitor of the essential Ca²⁺ pump SERCA2, insertional mutagenesis retrieved cell clones overexpressing SERCA2. With chemical mutagenesis, we identify six single amino acid substitutions in the known SERCA2-thapsigargin binding interface that confer drug resistance. In a second screen, we used the anti-cancer drug MG132/bortezomib (Velcade), which inhibits proteasome activity. Using chemical mutagenesis, we found 7 point mutations in the essential subunit Psmb5 that map to the bortezomib binding surface. Importantly, 4 of these had previously been identified in human tumors with acquired bortezomib resistance. Insertional mutagenesis did not identify Psmb5 in this screen, demonstrating the unique ability of chemical mutagenesis to identify relevant point mutations in essential genes.

Thus, chemical mutagenesis in haploid embryonic stem cells can define the interaction of toxic small molecules with essential proteins at amino acid resolution, fully mapping small molecule-protein binding interfaces.

Targeted Gene Knock Out Using Nuclease-Assisted Vector Integration: Hemi- and Homozygous Deletion of JAG1

Gene editing technologies are revolutionizing fields such as biomedicine and biotechnology by providing a simple means to manipulate the genetic makeup of essentially any organism. Gene editing tools function by introducing double-stranded breaks at targeted sites within the genome, which the host cells repair preferentially by Non-Homologous End Joining. While the technologies to introduce double-stranded breaks have been extensively optimized, this progress has not been matched by the development of methods to integrate heterologous DNA at the target sites or techniques to detect and isolate cells that harbor the desired modification. We present here a technique for rapid introduction of vectors at target sites in the genome that enables efficient isolation of successfully edited cells.

A versatile genetic tool: haploid cells

Haploid cells are excellent tools to study gene function as they contain a single copy of the genome and are thus unable to mask the effect of mutations. Recently, haploid embryonic stem cells, which are capable of self-renewal and potentially differentiating into other cell types despite having only one set of chromosomes, have been established in several species. These unique haploid cells allow us to seek recessive gene functions in mammals, and have had a profound influence on the field of genetic screening and drug target identification. In this review, we briefly introduce advances and breakthroughs in haploid cell line research and broadly discuss the versatile application thereof.

Targeted Genome Replacement via Homology-directed Repair in Non-dividing Cardiomyocytes

Although high-throughput sequencing can elucidate the genetic basis of hereditary cardiomyopathy, direct interventions targeting pathological mutations have not been established. Furthermore, it remains uncertain whether homology-directed repair (HDR) is effective in non-dividing cardiomyocytes. Here, we demonstrate that HDR-mediated genome editing using CRISPR/Cas9 is effective in non-dividing cardiomyocytes. Transduction of adeno-associated virus (AAV) containing sgRNA and repair template into cardiomyocytes constitutively expressing Cas9 efficiently introduced a fluorescent protein to the C-terminus of Myl2. Imaging-based sequential evaluation of endogenously tagged protein revealed that HDR occurs in cardiomyocytes, independently of DNA synthesis. We sought to repair a pathological mutation in Tnnt2 in cardiomyocytes of cardiomyopathy model mice. An sgRNA that avoided the mutated exon minimized deleterious effects on Tnnt2 expression, and AAV-mediated HDR achieved precise genome correction at a frequency of ~12.5%. Thus, targeted genome replacement via HDR is effective in non-dividing cardiomyocytes, and represents a potential therapeutic tool for targeting intractable cardiomyopathy.

CRISPR/Cas9: Transcending the Reality of Genome Editing

With the expansion of the microbiology field of research, a new genome editing tool arises from the biology of bacteria that holds the promise of achieving precise modifications in the genome with a simplicity and versatility that surpasses previous genome editing methods. This new technique, commonly named CRISPR/Cas9, led to a rapid expansion of the biomedical field; more specifically, cancer characterization and modeling have benefitted greatly from the genome editing capabilities of CRISPR/Cas9. In this paper, we briefly summarize recent improvements in CRISPR/Cas9 design meant to overcome the limitations that have arisen from the nuclease activity of Cas9 and the influence of this technology in cancer research. In addition, we present challenges that might impede the clinical applicability of CRISPR/Cas9 for cancer therapy and highlight future directions for designing CRISPR/Cas9 delivery systems that might prove useful for cancer therapeutics.

Phenotypic screening with primary neurons to identify drug targets for regeneration and degeneration

High-throughput, target-based screening techniques have been utilized extensively for drug discovery in the past several decades. However, the need for more predictive in vitro models of in vivo disease states has generated a shift in strategy towards phenotype-based screens. Phenotype based screens are particularly valuable in studying complex conditions such as CNS injury and degenerative disease, as many factors can contribute to a specific cellular response. In this review, we will discuss different screening frameworks and their relative utility in examining mechanisms of neurodegeneration and axon regrowth, particularly in cell-based in vitro disease models.

CRISPR-Cas9 directed knock-out of a constitutively expressed gene using lance array nanoinjection

Background

CRISPR-Cas9 genome editing and labeling has emerged as an important tool in biologic research, particularly in regards to potential transgenic and gene therapy applications. Delivery of CRISPR-Cas9 plasmids to target cells is typically done by non-viral methods (chemical, physical, and/or electrical), which are limited by low transfection efficiencies or with viral vectors, which are limited by safety and restricted volume size. In this work, a non-viral transfection technology, named lance array nanoinjection (LAN), utilizes a microfabricated silicon chip to physically and electrically deliver genetic material to large numbers of target cells. To demonstrate its utility, we used the CRISPR-Cas9 system to edit the genome of isogenic cells. Two variables related to the LAN process were tested which include the magnitude of current used during plasmid attraction to the silicon lance array (1.5, 4.5, or 6.0 mA) and the number of times cells were injected (one or three times).

Results

Results indicate that most successful genome editing occurred after injecting three times at a current control setting of 4.5 mA, reaching a median level of 93.77 % modification. Furthermore, we found that genome editing using LAN follows a non-linear injection-dose response, meaning samples injected three times had modification rates as high as nearly 12 times analogously treated single injected samples.

Conclusions

These findings demonstrate the LAN’s ability to deliver genetic material to cells and indicate that successful alteration of the genome is influenced by a serial injection method as well as the electrical current settings.

Generation and application of mammalian haploid embryonic stem cells

Haploid cells contain one set of chromosomes and are amenable for genetic analyses. In mammals, haploidy exists only in gametes. An intriguing question is whether haploid cells can be derived from gametes. Recently, by application of haploid cell enrichment using fluorescence-activated cell sorting, stable haploid embryonic stem cells (haESCs) have been successfully derived from oocyte-derived parthenogenetic and sperm-derived androgenetic embryos from several species. Whilst both parthenogenetic and androgenetic (AG)-haESCs enable whole-genome genetic screening at the cellular level, such as screening of drug resistance or disease-related genes, AG-haESCs, after intracytoplasmic injection into oocytes, can also be used to produce alive semi-cloned mice. Nevertheless, one major drawback associated with wild-type AG-haESCs is the very low birth rate of healthy semi-cloned mice. Of interest, after inhibiting the expression of two paternally imprinted genes (H19 and Gtl2) in AG-haESCs by removal of their differentially DNA methylated regions, double-knockout AG-haESCs can efficiently and stably support the generation of healthy semi-cloned pups. Importantly, double-knockout AG-haESCs are feasible for multiple genetic manipulations, followed by efficient generation of semi-cloned mice carrying multiple genetic traits; thus they could be used to validate candidate loci that have been identified in genome-wide association studies of multigenic diseases by generation of mouse models carrying multiple alterations. Of note, by combining a CRISPR-Cas9 library and double-knockout AG-haESCs, semi-cloned mice carrying different mutant genes can be efficiently generated in one step, enabling functional mutagenic screening in mice. HaESCs, therefore, provide a powerful tool for genetic analyses in mammals at both the cellular and organismal levels.

Generation of genetically modified mice using CRISPR/Cas9 and haploid embryonic stem cell systems

With the development of high-throughput sequencing technology in the post-genomic era, researchers have concentrated their efforts on elucidating the relationships between genes and their corresponding functions. Recently, important progress has been achieved in the generation of genetically modified mice based on CRISPR/Cas9 and haploid embryonic stem cell (haESC) approaches, which provide new platforms for gene function analysis, human disease modeling, and gene therapy. Here, we review the CRISPR/Cas9 and haESC technology for the generation of genetically modified mice and discuss the key challenges in the application of these approaches.

Multiplexed Targeted Genome Engineering Using a Universal Nuclease-Assisted Vector Integration System

Engineered nucleases are capable of efficiently modifying complex genomes through introduction of targeted double-strand breaks. However, mammalian genome engineering remains limited by low efficiency of heterologous DNA integration at target sites, which is typically performed through homologous recombination, a complex, ineffective and costly process. In this study, we developed a multiplexable and universal nuclease-assisted vector integration system for rapid generation of gene knock outs using selection that does not require customized targeting vectors, thereby minimizing the cost and time frame needed for gene editing. Importantly, this system is capable of remodeling native mammalian genomes through integration of DNA, up to 50 kb, enabling rapid generation and screening of multigene knockouts from a single transfection. These results support that nuclease assisted vector integration is a robust tool for genome-scale gene editing that will facilitate diverse applications in synthetic biology and gene therapy.

Transient Low-Temperature Effects on Propidium Iodide Uptake in Lance Array Nanoinjected HeLa Cells

Understanding environmental factors relative to transfection protocols is key for improving genetic engineering outcomes. In the following work, the effects of temperature on a nonviral transfection procedure previously described as lance array nanoinjection are examined in context of molecular delivery of propidium iodide (PI), a cell membrane impermeable nucleic acid dye, to HeLa 229 cells. For treatment samples, variables include varying the temperature of the injection solution (3C and 23C) and the magnitude of the pulsed voltage used during lance insertion into the cells (+5 V and +7 V). Results indicate that PI is delivered at levels significantly higher for samples injected at 3C as opposed to 23C at four different postinjection intervals (t = 0, 3, 6, 9 mins; p-value ≤ 0.005), reaching a maximum value of 8.3 times the positive control for 3 C/7 V pulsed samples. Suggested in this work is that between 3 and 6 mins postinjection, a large number of induced pores from the injection event close. While residual levels of PI still continue to enter the treatment samples after 6 mins, it occurs at decreased levels, suggesting from a physiological perspective that many lance array nanoinjection (LAN) induced pores have closed, some are still present.

Efficient generation of FVII gene knockout mice using CRISPR/Cas9 nuclease and truncated guided RNAs

We investigated the effects of 5'-end truncated CRISPR RNA-guided Cas9 nuclease (tru-RGN, 17/18 nucleotides) on genome editing capability in NIH/3T3 cells, and its efficiencies on generating Factor VII (FVII) gene-knockout (KO) mice. In cultured cells, RGNs on-target editing activity had been varied when gRNAs was truncated, higher at Site Two (tF7-2 vs. F7-2, 49.5 vs. 30.1%) while lower in other two sites (Site One, tF7-1 vs.F7-1, 12.1 vs. 23.6%; Site Three, tF7-3 vs.F7-3, 7.7 vs 10.9%) (P < 0.05). Out of 15 predicated off-target sites, tru-RGNs showed significantly decreased frequencies at 5 sites. By microinjecting tru-RGN RNAs into zygotes, FVII KO mice were generated with higher efficiency at Site Two (80.1 vs. 35.8%) and Site One (55.0 vs 3.7%) (P < 0.05), but not at Site three (39.4 vs 27.8%) (P > 0.05) when compared with standard RGN controls. Knockout FVII mice demonstrated a delayed prothrombin time and decreased plasma FVII expression. Our study first demonstrates that truncated gRNAs to 18 complementary nucleotides and Cas9 nucleases, can effectively generate FVII gene KO mice with a significantly higher efficiency in a site-dependent manner. In addition, the off-target frequency was much lower in KO mice than in cell lines via RGN expression vector-mediated genome editing.

Microscopy-Based High-Content Screening

Image-based screening is used to measure a variety of phenotypes in cells and whole organisms. Combined with perturbations such as RNA interference, small molecules, and mutations, such screens are a powerful method for gaining systematic insights into biological processes. Screens have been applied to study diverse processes, such as protein-localization changes, cancer cell vulnerabilities, and complex organismal phenotypes. Recently, advances in imaging and image-analysis methodologies have accelerated large-scale perturbation screens. Here, we describe the state of the art for image-based screening experiments and delineate experimental approaches and image-analysis approaches as well as discussing challenges and future directions, including leveraging CRISPR/Cas9-mediated genome engineering.

Genome Editing Using Mammalian Haploid Cells

Haploid cells are useful for studying gene functions because disruption of a single allele can cause loss-of-function phenotypes. Recent success in generating haploid embryonic stem cells (ESCs) in mice, rats, and monkeys provides a new platform for simple genetic manipulation of the mammalian genome. Use of haploid ESCs enhances the genome-editing potential of the CRISPR/Cas system. For example, CRISPR/Cas was used in haploid ESCs to generate multiple knockouts and large deletions at high efficiency. In addition, genome-wide screening is facilitated by haploid cell lines containing gene knockout libraries.