Nonsense and frameshift mutations, which predict premature termination of translation, often cause a dramatic reduction in the amount of transcript from the mutant allele (nonsense-mediated mRNA decay). In some genes, these mutations also influence RNA splicing and induce skipping of the exon that contains the nonsense codon. To begin to dissect how premature termination alters the metabolism of RNA from the COL1A1 gene, we studied nonsense and frameshift mutations distributed over exons 11-49 of the gene. These mutations were originally identified in 10 unrelated families with osteogenesis imperfecta (OI) type 1. We observed marked reduction in steady-state amounts of mRNA from the mutant allele in both total cellular and nuclear RNA extracts of cells from affected individuals, suggesting that nonsense-mediated decay of COL1A1 RNA is a nuclear phenomenon. Position of the mutation within the gene did not influence this observation. None of the mutations induced skipping of either the exon containing the mutation or, for the frameshifts, the downstream exons with the new termination sites. Our data suggest that nonsense and frameshift mutations throughout most of the COL1A1 gene result in a null allele, which is associated with the predictable mild clinical phenotype, OI type 1.