A single base mismatch was detected by single-strand conformation polymorphism (SSCP) of the collagen type III gene in a patient with Ehlers-Danlos syndrome type IV. The patient's fibroblasts secreted both normal and slowly migrating type III procollagen molecules. Two-dimensional CNBr peptide mapping suggested that the defect was localised in the CB9 peptide or the C-propeptide region of the alpha 1 (III)-chain. Analysis of a set of restriction-endonuclease-digested fragments of an amplified cDNA sequence encoding CB9, identified a single-strand conformation polymorphism and localized it within a region of 79 bp corresponding to the carboxyl-terminal end of the CB9 peptide of the alpha 1(III)-chain. DNA sequence analysis demonstrated that the patient was heterozygous for a point mutation converting G to T at base pair 3440 of the collagen alpha 1(III) cDNA resulting in the substitution of glycine with valine at amino acid position 1009 of the alpha 1(III)-chain. The mutation in this patient lies within a region of mutations at the carboxyl-terminal end of the type III collagen alpha-helix which all produce a severe "acrogeric" form of EDS IV.