The proposita described here was a 24-year-old woman with an acrogeric form of the Ehlers-Danlos syndrome including a massive dissecting aortic aneurysm. She was found to have a single-base mutation that substituted glutamic acid for glycine at amino acid position 1021 in the triple-helical domain of the type III procollagen. It is the most carboxy-terminal single-base mutation characterized to date in the COL3A1 gene. Analysis of medium and cell layer proteins from proposita's cultured skin fibroblasts showed that the mutant protein was poorly secreted, migrated more slowly on a polyacrylamide gel, and was partially unstable at +25 degrees C to brief digestion with trypsin.