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TRANSFORMING growth factor-beta (TGF-β) inhibits cell proliferation by inducing a Gl-phase cell cycle arrest¹. Normal progression through Gl is promoted by the activity of the cyclin-dependent protein kinases CDK4 and CDK6 (ref. 2), which are inhibited by the protein p16^INK4. We have isolated a new member of the p16^INK4 family, p15^INK4B. p15 expression is induced ∼30-fold in human keratinocytes by treatment with TGF-β, suggesting that pi5 may act as an effector of TGF-β-mediated cell cycle arrest. The gene encoding p15 is located on chromosome 9 adjacent to the p16 gene at a frequent site of chromosomal abnormality in human tumours (9p21).