The intestinal epithelium is maintained by a balance between proliferation, differentiation and death that occurs as cells migrate up the crypt-villus axis. Cell cycle regulators such as cyclins, cyclin-dependent kinases (Cdks) and Cdk inhibitory proteins are expressed in a distinct pattern along the crypt-villus structure, suggesting their role in controlling intestinal cells. This is supported by observations that these cell cycle proteins are regulated by growth factors, nutrients and cell-cell contact in cultured intestinal epithelial cells. One of the key regulators of intestinal cell proliferation and differentiation is transforming growth factor-beta, which is expressed in the gut epithelium.