Cover Story: Cluster of differentiation 9 (CD9) forms a complex with CD81, and is a member of the tetraspanin superfamily. Cd9 and Cd81 are highly expressed in breast cancer cells, but their expression in healthy mammary glands remains unclear. Horiguchi et al. reported expression of Cd9 and Cd81 in mammary epithelial cells, with expression levels correlating with mammary gland development (Horiguchi et al. Expression and functions of cluster of differentiation 9 and 81 in rat mammary epithelial cells, pp. 515–522). To examine the functional roles of CD9 and CD81, Horiguchi et al. knocked down Cd9 and Cd81 gene expression using small interfering RNAs (siRNAs) in isolated CD9-positive mammary epithelial cells. They found that siRNAs against Cd9 and Cd81 inhibited estrogen-induced mammary epithelial cell proliferation. These findings provide novel insights into mammary epithelial cell proliferation during pregnancy and lactation, as well as in pathological processes associated with breast cancer.