Nucleic acid sequence and oncogenic properties of the HZ2 feline sarcoma virus v-abl insert

Inhibition of v-Abl transformation in 3T3 cells overexpressing different forms of the Abelson interactor protein Abi-1

The abi-1 gene encodes a protein that binds and is phosphorylated by the Abelson protein tyrosine kinase. Constructs expressing a full-length abi-1 cDNA, and a smaller cDNA arising from an alternatively spliced form, were generated and tested for their effect on transformation of NIH3T3 cells by the Abelson murine leukemia virus. Overexpression of both forms of the protein strongly inhibited transformation by the wild-type P160 strain of the virus, but not by the non-interacting mutant P90A strain. The inhibition required the SH3 domain of Abi-1, suggesting that a direct interaction was required for the effect. Rare breakthrough P160 transformants of the Abi-1 overexpressing lines were found to have downregulated Abi-1 protein levels by a post-transcriptional mechanism.

The SH3 Domain Contributes to BCR/ABL-Dependent Leukemogenesis In Vivo: Role in Adhesion, Invasion, and Homing

To determine the possible role of the BCR/ABL oncoprotein SH3 domain in BCR/ABL-dependent leukemogenesis, we studied the biologic properties of a BCR/ABL SH3 deletion mutant (▵SH3 BCR/ABL) constitutively expressed in murine hematopoietic cells. ▵SH3 BCR/ABL was able to activate known BCR/ABL-dependent downstream effector molecules such as RAS, PI-3kinase, MAPK, JNK, MYC, JUN, STATs, and BCL-2. Moreover, expression of ▵SH3 BCR/ABL protected 32Dcl3 murine myeloid precursor cells from apoptosis, induced their growth factor-independent proliferation, and resulted in transformation of primary bone marrow cells in vitro. Unexpectedly, leukemic growth from cells expressing ▵SH3 BCR/ABL was significantly retarded in SCID mice compared with that of cells expressing the wild-type protein. In vitro and in vivo studies to determine the adhesive and invasive properties of ▵SH3 BCR/ABL-expressing cells showed their decreased interaction to collagen IV- and laminin-coated plates and their reduced capacity to invade the stroma and to seed the bone marrow and spleen. The decreased interaction with collagen type IV and laminin was consistent with a reduced expression of α2 integrin by ▵SH3 BCR/ABL-transfected 32Dcl3 cells. Moreover, as compared with wild-type BCR/ABL, which localizes primarily in the cytoskeletal/ membrane fraction, ▵SH3 BCR/ABL was more evenly distributed between the cytoskeleton/membrane and the cytosol compartments. Together, the data indicate that the SH3 domain of BCR/ABL is dispensable for in vitro transformation of hematopoietic cells but is essential for full leukemogenic potential in vivo.

The SH3 Domain Contributes to BCR/ABL-Dependent Leukemogenesis In Vivo: Role in Adhesion, Invasion, and Homing

To determine the possible role of the BCR/ABL oncoprotein SH3 domain in BCR/ABL-dependent leukemogenesis, we studied the biologic properties of a BCR/ABL SH3 deletion mutant (▵SH3 BCR/ABL) constitutively expressed in murine hematopoietic cells. ▵SH3 BCR/ABL was able to activate known BCR/ABL-dependent downstream effector molecules such as RAS, PI-3kinase, MAPK, JNK, MYC, JUN, STATs, and BCL-2. Moreover, expression of ▵SH3 BCR/ABL protected 32Dcl3 murine myeloid precursor cells from apoptosis, induced their growth factor-independent proliferation, and resulted in transformation of primary bone marrow cells in vitro. Unexpectedly, leukemic growth from cells expressing ▵SH3 BCR/ABL was significantly retarded in SCID mice compared with that of cells expressing the wild-type protein. In vitro and in vivo studies to determine the adhesive and invasive properties of ▵SH3 BCR/ABL-expressing cells showed their decreased interaction to collagen IV- and laminin-coated plates and their reduced capacity to invade the stroma and to seed the bone marrow and spleen. The decreased interaction with collagen type IV and laminin was consistent with a reduced expression of α2 integrin by ▵SH3 BCR/ABL-transfected 32Dcl3 cells. Moreover, as compared with wild-type BCR/ABL, which localizes primarily in the cytoskeletal/ membrane fraction, ▵SH3 BCR/ABL was more evenly distributed between the cytoskeleton/membrane and the cytosol compartments. Together, the data indicate that the SH3 domain of BCR/ABL is dispensable for in vitro transformation of hematopoietic cells but is essential for full leukemogenic potential in vivo.

Transduction of Notch2 in feline leukemia virus-induced thymic lymphoma

Feline leukemia virus (FeLV) is thought to induce neoplastic diseases in infected cats by a variety of mechanisms, including the transduction of host proto-oncogenes. While FeLV recombinants that encode cellular sequences have been isolated from tumors of naturally infected animals, the acquisition of an unrelated host gene has never been documented in an experimental FeLV infection. We isolated recombinant FeLV proviruses encoding feline Notch2 sequences from thymic lymphoma DNA of two cats inoculated with the molecularly cloned virus FeLV-61E. Four recombinant genomes were identified, three in one cat and one in the other. Each had similar but distinct transduction junctions, and in all cases, the insertions replaced most of the envelope gene with a region of Notch2 that included the intracellular ankyrin repeat functional domain. The product of the FeLV/Notch2 recombinant provirus was a novel, truncated 65- to 70-kD Notch2 protein that was targeted to the cell nucleus. This virally encoded Notch2 protein, which resembles previously constructed, constitutively activated forms of Notch, was apparently expressed from a subgenomic transcript spliced at the normal envelope donor and acceptor sequences. The data reported here implicate a nuclear, activated Notch2 protein in FeLV-induced leukemogenesis.

Abi-2, a novel SH3-containing protein interacts with the c-Abl tyrosine kinase and modulates c-Abl transforming activity

A protein has been identified that interacts specifically with both the Src homologous 3 (SH3) domain and carboxy-terminal sequences of the c-Abl tyrosine kinase. The cDNA encoding the Abl interactor protein (Abi-2), was isolated from a human lymphocyte library using the yeast two-hybrid system with the Abl SH3 domain as bait. Abi-2 binds to c-Abl in vitro and in vivo. Abi-2 is a novel protein that contains an SH3 domain and proline-rich sequences critical for binding to c-Abl. A basic region in the amino terminus of Abi-2 is homologous to the DNA-binding sequence of homeo-domain proteins. We show that Abi-2 is a substrate for the c-Abl tyrosine kinase. Expression of an Abi-2 mutant protein that lacks sequences required for binding to the Abl SH3 domain but retains binding to the Abl carboxyl terminus activates the transforming capacity of c-Abl. The properties of Abi-2 are consistent with a dual role as regulator and potential effector of the c-Abl protein and suggest that Abi-2 may function as a tumor suppressor in mammalian cells.

Abl-interactor-1, a novel SH3 protein binding to the carboxy-terminal portion of the Abl protein, suppresses v-abl transforming activity

A novel cellular protein, Abl-interactor-1 (Abi-1), which specifically interacts with the carboxy-terminal region of Abl oncoproteins, has been identified in a mouse leukemia cell line. The protein exhibits sequence similarity to homeotic genes, contains several polyproline stretches, and includes a src homology 3 (SH3) domain at its very carboxyl terminus that is required for binding to Abl proteins. The abi-1 gene has been mapped to mouse chromosome 2 and is genetically closely linked to the c-abl locus. The gene is widely expressed in the mouse, with highest levels of mRNA found in the bone marrow, spleen, brain, and testes. The Abi-1 protein coimmunoprecipitates with v-Abl and serves as a substrate for kinase activity. When overexpressed in NIH-3T3 cells, abi-1 potently suppresses the transforming activity of Abelson leukemia virus expressing the full-length p160v-abl kinase but does not affect the transforming activity of viruses expressing a truncated p90v-abl or v-src kinases. We suggest that the Abi-1 protein may serve as a regulator of Abl function in transformation or in signal transduction.

Carboxyl-terminal determinants of Abelson protein important for lymphoma induction

The carboxyl-terminal region of the Abelson protein is not absolutely required for Abelson virus transformation. However, Abelson virus strains encoding proteins missing portions of this region have a reduced ability to transform lymphoid cells in vitro and in vivo. One such strain, called P90A, is unique in that P90A-injected mice almost always develop tumors containing highly oncogenic variants that encode new forms of Abelson protein. In this work, we have examined the mechanism by which these variants are generated and used the variants to identify carboxyl-terminal protein sequences important for the induction of Abelson disease. Analysis of mice injected with helper-free P90A virus stocks demonstrates that the variants are generated during viral replication in vivo, probably as a consequence of error-prone reverse transcription. The sequence of the P90A viral genome reveals that a 19-base deletion is responsible for synthesis of the truncated Abelson protein. As a consequence of this mutation, 167 carboxyl-terminal amino acids normally found in the wild-type protein have been replaced by 33 amino acids derived from an alternative reading frame. Site-directed mutants show that the combination of the deletion and the P90A carboxyl terminus is required for the generation of variants. Thus, the particular structure of the P90A protein, not the specific residues lost or gained, alters the transforming potential of the Abelson protein. Finally, the sequence of the variants encoding smaller Abelson proteins reveals that as few as 452 v-abl-encoded amino acids are required for rapid induction of Abelson disease.

Differences in oncogenic potency but not target cell specificity distinguish the two forms of the BCR/ABL oncogene

Two forms of activated BCR/ABL proteins, P210 and P185, that differ in BCR-derived sequences, are associated with Philadelphia chromosome-positive leukemias. One of these diseases is chronic myelogenous leukemia, an indolent disease arising in hematopoietic stem cells that is almost always associated with the P210 form of BCR/ABL. Acute lymphocytic leukemia, a more aggressive malignancy, can be associated with both forms of BCR/ABL. While it is virtually certain that BCR/ABL plays a central role in both of these diseases, the features that determine the association of a particular form with a given disease have not been elucidated. We have used the bone marrow reconstitution leukemogenesis model to test the hypothesis that BCR sequences influence the ability of activated ABL to transform different types of hematopoietic cells. Our studies reveal that both P185 and P210 induce a similar spectrum of hematological diseases, including granulocytic, myelomonocytic, and lymphocytic leukemias. Despite the similarity of the disease patterns, animals given P185-infected marrow developed a more aggressive disease after a shorter latent period than those given P210-infected marrow. These data demonstrate that the structure of the BCR/ABL oncoprotein does not affect the type of disease induced by each form of the oncogene but does control the potency of the oncogenic signal.

Differences in oncogenic potency but not target cell specificity distinguish the two forms of the BCR/ABL oncogene

Two forms of activated BCR/ABL proteins, P210 and P185, that differ in BCR-derived sequences, are associated with Philadelphia chromosome-positive leukemias. One of these diseases is chronic myelogenous leukemia, an indolent disease arising in hematopoietic stem cells that is almost always associated with the P210 form of BCR/ABL. Acute lymphocytic leukemia, a more aggressive malignancy, can be associated with both forms of BCR/ABL. While it is virtually certain that BCR/ABL plays a central role in both of these diseases, the features that determine the association of a particular form with a given disease have not been elucidated. We have used the bone marrow reconstitution leukemogenesis model to test the hypothesis that BCR sequences influence the ability of activated ABL to transform different types of hematopoietic cells. Our studies reveal that both P185 and P210 induce a similar spectrum of hematological diseases, including granulocytic, myelomonocytic, and lymphocytic leukemias. Despite the similarity of the disease patterns, animals given P185-infected marrow developed a more aggressive disease after a shorter latent period than those given P210-infected marrow. These data demonstrate that the structure of the BCR/ABL oncoprotein does not affect the type of disease induced by each form of the oncogene but does control the potency of the oncogenic signal.

BCR first exon sequences specifically activate the BCR/ABL tyrosine kinase oncogene of Philadelphia chromosome-positive human leukemias

The c-abl proto-oncogene encodes a cytoplasmic tyrosine kinase which is homologous to the src gene product in its kinase domain and in the upstream kinase regulatory domains SH2 (src homology region 2) and SH3 (src homology region 3). The murine v-abl oncogene product has lost the SH3 domain as a consequence of N-terminal fusion of gag sequences. Deletion of the SH3 domain is sufficient to render the murine c-abl proto-oncogene product transforming when myristylated N-terminal membrane localization sequences are also present. In contrast, the human BCR/ABL oncogene of the Philadelphia chromosome translocation has an intact SH3 domain and its product is not myristylated at the N terminus. To analyze the contribution of BCR-encoded sequences to BCR/ABL-mediated transformation, the effects of a series of deletions and substitutions were assessed in fibroblast and hematopoietic-cell transformation assays. BCR first-exon sequences specifically potentiate transformation and tyrosine kinase activation when they are fused to the second exon of otherwise intact c-ABL. This suggests that BCR-encoded sequences specifically interfere with negative regulation of the ABL-encoded tyrosine kinase, which would represent a novel mechanism for the activation of nonreceptor tyrosine kinase-encoding proto-oncogenes.

BCR first exon sequences specifically activate the BCR/ABL tyrosine kinase oncogene of Philadelphia chromosome-positive human leukemias

The c-abl proto-oncogene encodes a cytoplasmic tyrosine kinase which is homologous to the src gene product in its kinase domain and in the upstream kinase regulatory domains SH2 (src homology region 2) and SH3 (src homology region 3). The murine v-abl oncogene product has lost the SH3 domain as a consequence of N-terminal fusion of gag sequences. Deletion of the SH3 domain is sufficient to render the murine c-abl proto-oncogene product transforming when myristylated N-terminal membrane localization sequences are also present. In contrast, the human BCR/ABL oncogene of the Philadelphia chromosome translocation has an intact SH3 domain and its product is not myristylated at the N terminus. To analyze the contribution of BCR-encoded sequences to BCR/ABL-mediated transformation, the effects of a series of deletions and substitutions were assessed in fibroblast and hematopoietic-cell transformation assays. BCR first-exon sequences specifically potentiate transformation and tyrosine kinase activation when they are fused to the second exon of otherwise intact c-ABL. This suggests that BCR-encoded sequences specifically interfere with negative regulation of the ABL-encoded tyrosine kinase, which would represent a novel mechanism for the activation of nonreceptor tyrosine kinase-encoding proto-oncogenes.

Activation of tyrosinase kinase and microfilament-binding functions of c-abl by bcr sequences in bcr/abl fusion proteins

Chronic myelogenous leukemia and one type of acute lymphoblastic leukemia are characterized by a 9;22 chronosome translocation in which 5' sequences of the bcr gene become fused to the c-abl proto-oncogene. The resulting chimeric genes encode bcr/abl fusion proteins which have deregulated tyrosine kinase activity and appear to play an important role in induction of these leukemias. A series of bcr/abl genes were constructed in which nested deletions of the bcr gene were fused to the c-abl gene. The fusion proteins encoded by these genes were assayed for autophosphorylation in vivo and for differences in subcellular localization. Our results demonstrate that bcr sequences activate two functions of c-abl; the tyrosine kinase activity and a previously undescribed microfilament-binding function. Two regions of bcr which activate these functions to different degrees have been mapped: amino acids 1 to 63 were strongly activating and amino acids 64 to 509 were weakly activating. The tyrosine kinase and microfilament-binding functions were not interdependent, as a kinase defective bcr/abl mutant still associated with actin filaments and a bcr/abl mutant lacking actin association still had deregulated kinase activity. Modification of actin filament functions by the bcr/abl tyrosine kinase may be an important event in leukemogenesis.

Activation of tyrosinase kinase and microfilament-binding functions of c-abl by bcr sequences in bcr/abl fusion proteins

Chronic myelogenous leukemia and one type of acute lymphoblastic leukemia are characterized by a 9;22 chronosome translocation in which 5' sequences of the bcr gene become fused to the c-abl proto-oncogene. The resulting chimeric genes encode bcr/abl fusion proteins which have deregulated tyrosine kinase activity and appear to play an important role in induction of these leukemias. A series of bcr/abl genes were constructed in which nested deletions of the bcr gene were fused to the c-abl gene. The fusion proteins encoded by these genes were assayed for autophosphorylation in vivo and for differences in subcellular localization. Our results demonstrate that bcr sequences activate two functions of c-abl; the tyrosine kinase activity and a previously undescribed microfilament-binding function. Two regions of bcr which activate these functions to different degrees have been mapped: amino acids 1 to 63 were strongly activating and amino acids 64 to 509 were weakly activating. The tyrosine kinase and microfilament-binding functions were not interdependent, as a kinase defective bcr/abl mutant still associated with actin filaments and a bcr/abl mutant lacking actin association still had deregulated kinase activity. Modification of actin filament functions by the bcr/abl tyrosine kinase may be an important event in leukemogenesis.

Temperature-sensitive mutants of Abelson murine leukemia virus deficient in protein tyrosine kinase activity

The effect of two missense mutations in abl on transformation by Abelson murine leukemia virus was evaluated. These mutations led to the substitution of a histidine for Tyr-590 and a glycine for Lys-536. Both changes gave rise to strains that were temperature dependent for transformation of both NIH 3T3 cells and lymphoid cells when expressed in the context of a truncated Abelson protein. In the context of the prototype P120 v-abl protein, the Gly-536 substitution generated a host range mutant that induced conditional transformation in lymphoid cells but had only a subtle effect on NIH 3T3 cells. The combination of both substitutions gave rise to a P120 strain that was temperature sensitive for both NIH 3T3 and lymphoid cell transformation. The Abelson proteins encoded by the temperature-sensitive strain displayed in vitro kinase activities that were reduced when compared with those of wild-type proteins. In vivo, levels of phosphotyrosine were reduced only at the restrictive temperature. Analysis of cells expressing either the wild-type P160 v-abl protein or the P210 bcr/abl protein and an Abelson protein encoded by a temperature-sensitive strain failed to correct this defect, suggesting either that tyrosine phosphorylation in vivo is an intramolecular reaction or that the protein encoded by the temperature-sensitive strain is a poor substrate for tyrosine phosphorylation in vivo. These results raise the possibility that tyrosine phosphorylation of Abelson protein plays a role in transformation.

Activation of murine c-abl protooncogene: effect of a point mutation on oncogenic activation

Activation of the c-abl protooncogene occurs in Abelson murine leukemia virus, in Hardy-Zuckerman 2 feline sarcoma virus, and during the chromosomal translocations that generate BCR-ABL gene fusion products. To study the molecular mechanism involved in the c-abl activation, we have created a series of modifications in murine c-abl and assayed these constructs for oncogenic activity using the NIH 3T3 cell transformation assay. Our results show that amino-terminal deletions are sufficient for oncogenic activation of c-abl and high levels of oncogenic activities were generated by a deletion of 114 codons from the 5' end that deleted the SH3 region. A deletion of 53 codons from the 5' end (inclusive of deletions seen in Hardy-Zuckerman 2 feline sarcoma virus and BCR-ABL gene products) that retains the SH3 region of c-abl resulted in the generation of low levels of transforming activity. This transforming potential was substantially increased with the introduction of a G----A point mutation in codon 832 that is present in v-abl. The point mutation was found to affect the secondary structure and the tyrosine kinase activity of the mutant gene products.

Conservation of function of Drosophila melanogaster abl and murine v-abl proteins in transformation of mammalian cells

The Drosophila melanogaster abl and the murine v-abl genes encode tyrosine protein kinases (TPKs) whose amino acid sequences are highly conserved. To assess functional conservation between the two gene products, we constructed Drosophila abl/v-abl-chimeric Abelson murine leukemia viruses. In these chimeric Abelson murine leukemia viruses, the TPK and carboxy-terminal regions of v-abl were replaced with the corresponding regions of D. melanogaster abl. The chimeric Abelson murine leukemia viruses were able to mediate morphological and oncogenic transformation of NIH 3T3 cells and were able to abrogate the interleukin-3 dependence of a lymphoid cell line. We also found that a virus that contained both TPK and carboxy-terminal Drosophila abl regions had no in vitro transforming activity for primary bone marrow cells and lacked the ability to induce tumors in susceptible mice. A virus that replaced only a portion of the v-abl TPK region with that of Drosophila abl had low activity in in vitro bone marrow transformation and tumorigenesis assays. These results indicate that the transforming functions of abl TPKs are only partially conserved through evolution. These results also imply that the TPK region of v-abl is a major determinant of its efficient lymphoid cell-transforming activity.

Primary structure of c-kit: relationship with the CSF-1/PDGF receptor kinase family--oncogenic activation of v-kit involves deletion of extracellular domain and C terminus

The protein kinase domains of v-kit, the oncogene of the acute transforming feline retrovirus HZ4-FeSV (HZ4-feline sarcoma virus), CSF-1R (macrophage colony stimulating factor receptor) and PDGFR (platelet derived growth factor receptor) display extensive homology. Because of the close structural relationship of v-kit, CSF-1R and PDGFR we predicted that c-kit would encode a protein kinase transmembrane receptor (Besmer et al., 1986a; Yarden et al., 1986). We have now determined the primary structure of murine c-kit from a DNA clone isolated from a brain cDNA library. The nucleotide sequence of the c-kit cDNA predicts a 975 amino acid protein product with a calculated mol. wt of 109.001 kd. It contains an N-terminal signal peptide, a transmembrane domain (residues 519-543) and in the C-terminal half the v-kit homologous sequences (residues 558-925). c-kit therefore contains the features which are characteristic of a transmembrane receptor kinase. Comparison of c-kit, CSF-1R and PDGFR revealed a unique structural relationship of these receptor kinases suggesting a common evolutionary origin. The outer cellular domain of c-kit was shown to be related to the immunoglobulin superfamily. The sites of expression of c-kit in normal tissue predict a function in the brain and in hematopoietic cells. N-terminal sequences which include the extracellular domain and the transmembrane domain as well as 50 amino acids from the C-terminus of c-kit are deleted in v-kit. These structural alterations are likely determinants of the oncogenic activation of v-kit.(ABSTRACT TRUNCATED AT 250 WORDS)

The viral and cellular forms of the Abelson (abl) oncogene

The precision of molecular biology has allowed a better definition of the components of the Abelson system. We know the gene structures and gene products for the cellular and viral forms of this family of related tyrosine kinases. However, many basic issues first identified in the early biological observations of Abelson, Rabstein, and others remain unanswered. The precise pathway for transformation in biochemical terms remains unknown for Ab-MLV and all of its relatives. Relatively little can be said to explain the preferential growth stimulation for certain hematopoietic cell types by the viral and other altered forms of the oncogene, and no clear insights into the function of the normal cellular forms of the abl oncogene are available. Future progress will certainly depend on the intensive efforts by many workers in the broader field of cellular growth control mechanisms.

Nucleotide sequence of testis-derived c-abl cDNAs: implications for testis-specific transcription and abl oncogene activation

The c-abl gene codes for a protein-tyrosine kinase and is expressed in most examined murine cell types as two distinct mRNA species of 5.5 kilobases (kb) and 6.5 kb. In mouse testis, an additional species of 4.0 kb is expressed in very high levels. To study the interrelationship between various c-abl transcripts and to compare their sequence with the v-abl transcript, we prepared c-abl-specific cDNA clones from mouse testis and determined the complete nucleotide sequence of the 4.0-kb cDNA that appears to be the reverse transcript of the testis-specific mRNA. In addition, we have determined the 3' sequence of an additional clone derived from the larger mRNA species that is expressed in somatic as well as germ-line cells. These cDNA sequences have been compared with the v-abl sequences to understand the mechanism of activation of this oncogene. The results demonstrate that (i) testis-specific c-abl mRNAs arise as a result of 3' truncation, and (ii) the v-abl gene has arisen from its cellular homologue as a result of an extensive deletional/mutational process.

Role of the ABL oncogene tyrosine kinase activity in human leukaemia

A great deal of information has emerged over the past decade regarding the gene structures and corresponding protein products of the cellular and transformation-associated forms of the ABL tyrosine kinase family. Many reports have also detailed the biological effects of these proteins (particularly the viral ABL forms) on a broad range of cell types. However, in spite of all these research efforts, the precise role of the ABL gene in normal and neoplastic growth remains to be determined. To elucidate the mechanism of action of normal and altered ABL proteins, it is imperative to identify their relevant cellular substrates and establish the role of the ABL target proteins in transformation and normal cellular growth. The availability of temperature-sensitive ABL proteins, coupled with the use of sensitive anti-phosphotyrosine antibodies, should be useful in this respect. Purification of enzymatically active, intact forms of the ABL proteins produced in insect cells by employing baculovirus expression vectors should permit direct comparison of the biochemical properties and tertiary structures of the various members of the ABL protein kinase family. Such studies will aid in understanding the nature of the alteration of ABL which results in the activation of its transforming potential. Furthermore, the availability of purified ABL proteins should permit examination of interactions of ABL with other growth-regulatory proteins, such as growth factor receptors. It has been shown that transformation-associated ABL proteins interact with the IL-3, IL-2 and GM-CSF growth-factor pathways. These and other components of the cellular signalling pathways are potential ABL targets. The elucidation of ABL function by a variety of approaches such as those described above will ultimately aid in the development of far-reaching therapeutic treatments for at least two forms of human leukaemia: Ph positive CML and Ph positive ALL.