Caveolin, an integral membrane protein, is a principal component of caveolae membranes in vivo. Two isoforms of caveolin have been identified: a slower migrating 24-kDa species (-isoform) and a faster migrating 21-kDa species (-isoform). Caveolins interact with multiple signaling molecules, such as the G-protein alpha subunit, tyrosine kinase receptors, PKCs, Src family tyrosine kinases and eNOS. Caveolin-1 has been implicated in the pathogenesis of mammary epithelial cell hyperplasia.