C-myb encodes a transcriptional activator that is essential for the development of the hematopoietic system but appears to lack major roles in non-hematopoietic cells. The identification of two conserved myb-related genes, designated A-myb and B-myb, has raised the possibility that these genes are functional equivalents of c-myb in non-hematopoietic cells. Here, we report the isolation and preliminary characterization of the mouse A-myb gene. Mouse A-myb maps to the proximal region of chromosome 1 and encodes a transcriptional activator with properties similar to those of the c-myb and v-myb proteins. During embryo-genesis A-myb is predominantly expressed in several regions of the developing central nervous system (CNS) and the urogenital ridge. Expression in the CNS is confined to the neural tube, the hindbrain, the neural retina and the olfactory epithelium, and coincides with the presence of proliferating immature neuronal precursor cells. In the adult mouse, A-myb is expressed during the early stages of sperm cell differentiation and in B lymphocytes located in germinal centers of the spleen. Taken together, these results suggest a role for A-myb in the proliferation and/or differentiation of neurogenic, spermatogenic and B-lymphoid cells.