The three members of the jun proto-oncogene family c-jun, jun b and jun D were mapped on the mouse chromosome by in situ hybridization. The c-jun locus is on chromosome 4 subregion C5----C7, whereas jun B and jun D are co-localized on chromosome 8 subregion C. RFLP analysis of interspecific hybrids confirmed the mapping of jun B and D and showed that they are situated about 7.3 +/- 3.5 cM apart. Thus despite their possible origin from a single ancestral gene they are not closely linked on the chromosome. Using the same probes, we showed that the human genome also contains sequences homologous to the mouse jun B and jun D. They are located on human chromosome 19 p13.2, a region that may be involved in chromosomal translocation in acute lymphocytic leukemia (ALL), acute nonlymphocytic leukemia (ANLL) and malignant melanoma (MEL). Finally, the present data identify a new segmental homology between mouse and human chromosomes.