The c Abl proto oncogene encodes a protein tyrosine kinase that is located in the cytoplasm and nucleus. In chronic myelogenous leukemia and in a subset of acute lymphoblastic leukemias, the c Abl proto oncogene undergoes a (9;22) chromosomal translocation producing a novel rearranged chromosome (the Philadelphia chromosome) As the result of the fusion of c Abl sequences from chromosome 9 to the Bcr gene on chromosome 22. The molecular consequence of this translocation is the generation of a chimeric Bcr/Abl mRNA encoding activated Abl protein tyrosine kinase.