In the Commentary titled "Genetic Epidemiology and Congenital Malformations: From the Chromosome to the Crib," published in the April issue of the ARCHIVES (2002;156:315-320), there was an error in a statement on page 317 in the second and third sentences in the "Twin Studies" subsection. The sentences should have read as follows: "Twin studies are based on the premise that monozygotic twins share all of their genes, whereas dizygotic twins share, on average, half of their genes. Thus, if a birth defect resulted exclusively from fully penetrant genetic factors, monozygotic twins would share the malformation in 100% of twins studied, whereas dizygotic twins would be expected to share the malformation in 50% of twins studied."