The human c-mos proto-oncogene is located on chromosome 8 at band q22, close to the breakpoint in the t(8;21) (q22;q22) chromosome rearrangement. This translocation is associated with acute myeloblastic leukemia, subgroup M2. The c-myc gene, another proto-oncogene, has been mapped to 8q24. The breakpoint at 8q22 separates these genes, as determined by in situ hybridization of c-mos and c-myc probes. The c-mos gene remains on the 8q-chromosome and the c-myc gene is translocated to the 21q+ chromosome. Southern blot analysis of DNA from bone marrow cells of four patients with this translocation showed no rearrangement of c-mos.