p57^KIP2 is a potent tight-binding inhibitor of several G₁ cyclin/Cdk complexes, and is a negative regulator of cell proliferation^1,2. The gene encoding p57^KIP2 is located at 11p15.5 (ref. 2), a region implicated in both sporadic cancers and Beckwith-Wiedemann syndrome, a cancer-predisposing syndrome, making it a tumour-suppressor candidate. Several types of childhood tumours including Wilms' tumour, adrenocortical carcinoma and rhabdomyosarcoma exhibit a specific loss of maternal 11p15 alleles, suggesting that genomic imprinting^3–8 is involved^9–12. Genetic analysis of the Beckwith-Wiedemann syndrome indicated maternal carriers, as well as suggesting a role of genomic imprinting¹³. Previously, we and others demonstrated that p57^KIP2 is imprinted and that only the maternal allele is expressed in both mice and humans^14–16. Here we describe p57^KIP2 mutations in patients with Beckwith-Wiedemann syndrome. Among nine patients we examined, two were heterozygous for different mutations in this gene -- a missense mutation in the Cdk inhibitory domain resulting in loss of most of the protein, and a frameshift resulting in disruption of the QT domain. The missense mutation was transmitted from the patient's carrier mother, indicating that the expressed maternal allele was mutant and that the repressed paternal allele was normal. Consequently, little or no active p57^KIP2 should exist and this probably causes the overgrowth in this BWS patient.