COL4A4 and Alport syndrome: In particular, mutations in the type IV collagen α5 chain gene (COL4A5) are responsible for the X-linked form of the disease, which accounts for ~85% of the patients and mutations in the type IV collagen α3 or α4 chain gene (COL4A3 or COL4A4) lead to the autosomal form of the Alport syndrome [2].