The transforming growth factor β (TGF-β) receptors are an important class of tumor suppressor gene. TGF-β markedly inhibits the growth of many epithelial cell types; whereas in contrast, cancers of many different tissue types are commonly TGF-β resistant (10 ). Many cancer cell lines are resistant to the growth suppressive effects of TGF-β and display evidence of disruption of TGF-β signal transduction (10 ,4 ,10 ). To date, this resistance appears to often result from mutations in the TGF-β receptors or thesmadfamily of TGF-β signaling proteins (5 ,11 ,12 ,14 ,16 ).
