COL4A5 and renal fibrosis: XLAS is characterized by mutations in COL4A5, the gene encoding the α5(IV) chain; in most men with XLAS, the basement membrane lacks the expression of α3α4α5 and α5α5α6 trimers.2) The absence of α3α4α5 trimers in the glomerular basement membrane induces structural and functional abnormalities of the glomerulus, which could ultimately lead to renal fibrosis.