Molecular analysis of acute promyelocytic leukemia breakpoint cluster region on chromosome 17. Acute promyelocytic leukemia (APL; FAB M3) is characterized by a predominance of malignant promyelocytes that carry a reciprocal translocation between the long arms of chromosomes 15 and 17, t(15;17) (q22;q11.2-q12). This translocation has become diagnostic for APL, as it is present in almost 100 percent of cases. A Not I linking clone was used to detect this translocation initially on pulsed-field gel electrophoresis and subsequently with conventional Southern (DNA) analysis. The breakpoints in ten APL cases examined were shown to cluster in a 12-kb region of chromosome 17, containing two CpG-rich islands. The region is the first intron of the retinoic acid receptor alpha gene (RARA). 