Chromosome rearrangements affecting band 3q21, namely, the inv(3)(q21q26), the t(3;3)(q21;q26), and the t(1;3)(p36;q21), are associated with a particularly poor prognosis in myeloid leukemia or myelodysplasia. Originally, inv(3) and t(3;3) breakpoints have been reported to cluster in a region (breakpoint cluster region, BCR) of approximately 30 kb, which is located centromeric and downstream of the ribophorin I (RPN-I) gene. More recently, we established a PAC contig that includes the 3q21 BCR, and used these PAC clones to map breakpoints in patient samples by both metaphase and interphase fluorescence in situ hybridization (FISH) analysis. A significant proportion of inv(3) and t(3;3) breakpoints was located at sometimes considerable distances centromeric of the originally described BCR, in a region recently also implicated in t(1;3) rearrangements. These breakpoints may thus define a second, centromeric BCR (BCR-C), or extend the original 3q21 BCR to a size of approximately 100 kb. Activation of the EVI-1 gene in 3q26 by regulatory sequences of the housekeeping gene RPN-I has been suggested as a leukemogenic mechanism in patients with inv(3) and t(3;3). However, despite a number of characteristics that make EVI-1 an attractive candidate oncogene, its biological properties fail to fully explain the phenotype of leukemias carrying 3q rearrangements. Several additional candidate genes have been identified in or near the 3q21 breakpoint region, but their possible contribution to the characteristics of leukemias with 3q21 rearrangements remains to be explored.