Transforming growth factor beta (TGF-β1) is a stable, multifunctional polypeptide growth factor. While specific receptors for this protein have been found on almost all mammalian cells examined, the effect of the molecule varies depending on the cell type and growth conditions. TGF-β is an anti-proliferation factor in normal cells. It increases the synthesis of p15 and p21, which can block the cyclin:CDK complex, and causes cells to stop at G1 phase. In cancer cells, TGF-β signaling is altered and TGF-β no longer stops cell proliferation. TGF-β1, is only one member of a family of regulatory proteins consisting of a number of factors distantly related to TGF-β1 (30-40% sequence identity), including the activins, inhibins, and bone morphogenetic proteins (BMPs), and a number of more closely related proteins (70-80% sequence identity) designated TGF-β1, TGF-β2, TGF-β3, TGF-β4, and TGF-β5. Human and mouse TGF-β1 share 99% gene homology and the mature form of TGF1beta in the two species are identical.