The v-myb oncogene, like its cellular progenitor c-myb, encodes a short-lived nuclear protein involved in processes affecting growth and differentiation in a number of cell types. Fusion proteins, in which v-myb sequences are linked to the DNA binding domain of the yeast transcriptional activator GAL4, can activate transcription from a reporter gene linked in cis to a GAL4 binding site. The domain of v-myb responsible for transcriptional activation is located between residues 204 and 254, and is both necessary and sufficient for activation. Intact v-myb and c-myb proteins can also activate transcription, via a myb binding site linked in cis to a reporter gene. A v-myb protein bearing a deletion in the activator domain is no longer capable of stimulating transcription.