AbstractWe have determined that a man, ascertained because he fathered a child with lethal osteogenesis imperfecta (OI) with each of two partners, is mosaic in both his germline and somatic tissues for a mutation in the COL1A2 gene which encodes the proα2(I) chain of type I procollagen. His dermal fibroblasts were previously shown to synthesize a population of cysteine‐containing α2(I) chains that were posttranslationally overmodified. DNA sequence analysis of COL1A2 cDNAs demonstrated that the cysteine‐containing chain resulted from a point mutation (G to T) in the first position of the codon for the glycine at residue 472 of the triple helical domain. Genomic DNA from the one available affected infant contained the mutant and normal COL1A2 alleles in equal proportion. Examination of DNA from several tissues of the father showed that the mutant allele was present in approximately 40% of his sperm, 80% of his lymphocytes, and nearly 100% of his dermal fibroblasts. Despite the high level of mosaicism detected in somatic tissues, the only phenotypic manifestation of OI in the proband was that he was shorter than his unaffected male relatives and had mild dentinogenesis imperfecta. Thermal stability of type I collagen molecules containing the substitution was decreased, but to a lesser extent than for a nonlethal cysteine for glycine substitution at residue 259 of α2(I), indicating that this measure of molecular stability may be of limited use in explaining the pathogenesis of osteogenesis imperfecta. © 1992 Wiley‐