Mice lacking cyclin D1 have been generated by gene targeting in embryonic stem cells. Cyclin D1-deficient animals develop to term but show reduced body size, reduced viability, and symptoms of neurological impairment. Their retinas display a striking reduction in cell number due to proliferative failure during embryonic development. In situ hybridization studies of normal mouse embryos revealed an extremely high level of cyclin D1 in the retina, suggesting a special dependence of this tissue on cyclin D1. In adult mutant females, the breast epithelial compartment fails to undergo the massive proliferative changes associated with pregnancy despite normal levels of ovarian steroid hormones. Thus, steroid-induced proliferation of mammary epithelium during pregnancy may be driven through cyclin D1.

Cyclin-dependent kinases (Cdks} are positive regulators of cell proliferation, whereas Cdk inhibitors {CKIs) inhibit proliferation. We describe a new CKI, p57 v'w2, which is related to p21 cn'1 and p27 gxP1. p57 gn'2 is a potent, tight-binding inhibitor of several G~ cyclin/Cdk complexes, and its binding is cyclin dependent.Unlike CIP1, KIP2 is not regulated by p53. Overexpression of p57 gn'e arrests cells in G~. p57 me2 proteins have a complex structure. Mouse p57/~n'2 consists of four structurally distinct domains: an amino-terminal Cdk inhibitory domain, a proline-rich domain, an acidic-repeat region, and a carboxy-terminal domain conserved with p27gn'L Human p57 ~2 appears to have conserved the amino-and carboxy-terminal domains but has replaced the internal regions with sequences containing proline-alanine repeats. In situ hybridization during mouse embryogenesis revealed that KIP2 mRNA displays a striking pattern of expression during development, showing high level expression in skeletal muscle, brain, heart, lungs, and eye. Most of the KIP2-expressing cells are terminally differentiated, suggesting that p57 ~n'2 is involved in decisions to exit the cell cycle during development and differentiation. Human KIP2 is located at 11p15.5, a region implicated in both sporadic cancers and Beckwith-Wiedemann syndrome, a familial cancer syndrome, marking it as a candidate tumor suppressor. The discovery of a new member of the p21 cn'l inhibitor family with novel structural features and expression patterns suggests a complex role for these proteins in cell cycle control and development.