The Philadelphia (Ph') chromosome is an abnormal chromosome 22 generated by a recombination event between chromosome 22 and 9; it is found in the leukemic cells of over 95% of all chronic myelogenous leukemia (CML) patients and is diagnostic for the disease. A significant percentage of acute lymphocytic leukemia (ALL) patients also have a Ph'-chromosome in their leukemic cells. In CML, the c-abl oncogene (ABL) is located at the breakpoint on chromosome 9 and the bcr gene at the breakpoint on chromosome 22; breakpoints in the bcr gene are distributed over a region of less than 5.8 kb, the "breakpoint cluster region" (bcr). However, in Ph'-positive ALL, not all patients have a breakpoint within this region of chromosome 22. The major objectives of this project are the identification and characterization of the breakpoints on chromosome 22 in those Ph'-positive ALL patients lacking a breakpoint in the bcr. To this end, a large number of Ph'-positive ALL DNAs will be examined with probes from the bcr by Southern blot analysis; those DNAs lacking a bcr breakpoint will be examined more closely; the 5' region of the bcr gene will be cloned and characterized; repeat- free probes will be used to identify breakpoints 5' in the gene and to clone the breakpoint fragments of at least one patient. The possibility of clustering of breakpoints 5' within the gene in different DNAs will be examined by Southern blot analysis. If specimens are found with a breakpoint outside the bcr gene, the breakpoint sequences on chromosome 22 will be identified using a cDNA cloning strategy: cDNA libraries constructed from poly(A)+ RNA of these patients will be screened with an ABL probe and putative chimeric cDNAs will be identified by restriction enzyme analysis. These experiments should provide insight into the possible role of the Philadelphia chromosome and of ABL in the genesis and/or maintenance of Ph'-positive ALL. In addition, the proposed research might shed light on the molecular significance of the bcr part of the bcr/ ABL protein, which represents the hallmark of CML.