AbstractA single‐base mutation in intron 37 of the gene for type III procollagen (COL3A1) was found in a proband with the type IV variant of Ehlers—Danlos syndrome. Probe‐protection experiments with S1 nuclease and RNA from fibroblasts incubated at 37°C demonstrated that about 35% of the total mRNA or about 70% of the mRNA from mutated allele was spliced by exon skipping. The effects of the mutation were temperature‐sensitive in that the amount of RNA from the mutated allele that was spliced by exon skipping was 87.1 ± 7.7% at 31°C, 70.1 ± 6.5% at 37°C, and 85.4 ± 11.1% at 42°C. The effects of temperature on aberrant RNA splicing were, therefore, the reverse of those reported for four previous mutants incollagen genes. The increase in abnormal RNA splicing when the temperature was raised from 31° to 37°C seen with previously reported mutants suggested that RNA–RNA hybridization of U1snRNA to the 5′‐splice site in the substrate may be limiting in the processing of transcripts from the mutated alleles, since RNA–RNA hybridizations become less favorable at higher temperatures. The decrease in abnormal RNA splicing seen here when the temperature was raised from 31° to 37°C suggested that protein–RNA or protein–protein binding steps become rate limiting with the G+5mutation in intron 37 of the COL3A