Beckwith-Wiedemann syndrome is a human congenital disorder characterized by a wide variety of growth abnormalities, including developmental defects and predisposition to certain tumors. Genetic evidence has suggested a role for p57KIP2, a member of a family of cell cycle inhibitory genes, in Beckwith-Wiedemann syndrome. Two independent groups have reported the generation and characterization of mice lacking functional p57KIP2. These mice demonstrate a number of abnormal phenotypes which overlap with, although do not completely recapitulate, Beckwith-Wiedemann syndrome. These findings advance the molecular characterization of a human disorder, and provide insight into the interplay between regulation of cell division and development.