In a large Italian family with adult-onset Alport syndrome, molecular analysis of the COL4A5 gene, which encodes the alpha 5(IV)-chain of glomerular basement membrane collagen, revealed a GGC-->AGC change in exon 38, resulting in substitution of a serine for a glycine in position 1143 of the polypeptide chain, between interruptions 19 and 20 of the triple helical domain. The mutation leads to loss of a restriction site for the enzyme Msp I, and could thus be easily recognized in several female and male relatives. Among relatives of both sexes who carried the same mutation, the clinical phenotype of Alport syndrome was variable as for the onset of renal failure and the presence of associated ear and eye abnormalities.