p57^Kip2 is a potent, tight-binding inhibitor of several G1 cyclin/Cdk complexes, and its binding is cyclin dependent. Its over-expression leads to arrest of the cell in G1 phase. Human p57^Kip2 appears to have conserved the amino- and carboxy-terminal domains but has replaced the internal regions with sequences containing proline-alanine repeats. Expression patterns suggest a complex role for p57^Kip2 cell cycle control and development. Because complete hydatidiform moles lack a maternal genome, p57^Kip2 immunostaining is correspondingly absent, whereas hydropic abortuses and partial moles show positive staining. p57^Kip2 is a marker distinguishing complete hydatidiform moles (negative) from partial moles (positive).