Mammary gland development and differentiation is regulated by a number of growth factors and hormones. Milk protein gene expression represents a hallmark of functional mammary epithelial differentiation and is coordinated by the lactogenic hormone prolactin and glucocorticoids. To date, few 'early-response' genes transcriptionally activated by lactogenic hormones have been described. We have used representational difference analysis (RDA) to search for lactogenic-responsive genes in SCp2 mouse mammary epithelial cells. One of the cDNAs identified encoded the DNA-binding protein Taxreb107, originally identified as a HTLV-I Tax responsive element binding protein. Increased Taxreb107 expression was confirmed following prolactin and dexamethasone-induced differentiation of SCp2 and HC11 mammary epithelial cells. Taxreb107 RNA levels were developmentally regulated in the mouse mammary gland, where levels increased substantially during mid- and late pregnancy and persisted during lactation. Overexpression of an antisense Taxreb107 cDNA construct or antisense oligonucleotide in HC11 mammary epithelial cells attenuated milk protein gene expression following prolactin and dexamethasone treatment. These findings indicate a role for Taxreb107 as a lactogenic hormone-responsive gene during differentiation of the mammary gland.