The myb family of genes includes the virally encoded v-myb oncogene, the c-myb protooncogene from which it is derived, and two structurally related genes, B-myb and A-myb. C-myb is most highly expressed in hematopoietic cells and its oncogenic activation leads to transformation, primarily of myeloid cells. Several lines of evidence suggest that c-myb functions in regulating both the proliferation and differentiation of hematopoietic cells of different lineages, including early progenitors. The mechanisms of action and the regulation of expression of c-myb and v-myb will be described. The possible role of the B-myb and A-myb gene products will also be discussed.