A partial-length A-myb complementary DNA recently cloned by low-stringency hybridization with a c-myb probe to complementary DNA libraries derived from human cell lines showed a high degree of homology with the DNA-binding domain of c-myb and B-myb, suggesting that A-myb also encoded a DNA-binding protein. We report here the sequence of the entire coding region of A-myb complementary DNA and show that the full-length GST-A-myb fusion protein or a truncated derivative corresponding only to the putative DNA-binding domain interacts specifically with Myb-binding sites of the c-myb responsive promoters, MIM-1 and CD34. In transient transfection assays, A-myb transactivated the bound promoters. These results suggest that, analogous to the other members of the Myb family, the A-myb gene encodes a bona fide transactivator. The distinct function of A-myb might derive from its pattern of expression and/or its relative potency as a transactivator of myb target genes.