Ehlers-Danlos syndrome type IV (EDS IV) was diagnosedin a 22 year old man because of typical clinical signs andfamily history. His father had died aged 32 years frominternal haemorrhage. Biochemical studies showed that (a)skin fibroblasts from this patient secreted reduced amountsof type III procollagen, (b) in a major portion of type IIIprocollagen the triple helical structure extended from theC-terminus only to three-quarters of the normal length, (c)the abnormal type III procollagen molecules were unstableand poorly secreted, and (d) in the presence of dextran,the abnormal molecules which were secreted did notundergo further processing as did their normal counterparts.Immunoblotting experiments (antiserum kindlyprovided by Dr R Timpl) showed that two populations oftype III proto-procollagen chains were produced by thepatient's cells, one of normal and another of smaller size.Similarly, Northern blots probed with cDNAs for type IIIcollagen (kindly provided by Drs E Vuorio and RDalgleish) showed that two species of mRNA were presentin equal quantities, a normal sized mRNA and a mutantmRNA which was about 600 bp shorter. We conclude thatone mutant allele (dominant inheritance) at the locus fortype III collagen (COL3AJ) codes for an abnormally shortmRNA which results in the synthesis of shortened type IIIprocollagen chains. The mutant chains are incorporatedinto trimers, but trimers with one or more mutant chainsare biochemically and functionally crippled. These findingsprovide direct evidence of a mutation at the COL3AI locusin EDS IV.