Several Wnt genes are expressed in the postnatal mouse mammary gland and are thought to be involved in mammary gland development. Ectopic expression of Wnt-1, which is not normally expressed in the mammary gland, drives the formation of a pre-neoplastic hyperplasia. Cell culture-based assays have shown that Wnt-1 and some mammary-expressed Wnts transform C57MG cells. This has led to the suggestion that Wnt-1 functions as an oncogene through the inappropriate activation of developmental events that are normally controlled by the 'transforming' class of Wnts. In this study, Wnt-7b was expressed in vivo using recombinant retroviruses. Wnt-7b did not alter normal mammary gland development despite having similar effects to Wnt-1 in cell culture. We conclude that the in vitro classification of Wnts as 'transforming' does not correlate with the transformation in vivo. To facilitate the analysis of Wnt-expression, a lacZ-containing, bicistronic recombinant retrovirus was developed. Immunohistochemistry and electron microscopy identified retrovirally transduced myoepithelial and luminal epithelial cells in normal and hyperplastic tissues. The distribution of transduced cells in mammary outgrowths was consistent with current models of mammary stem cell identity.