COL4A3 and X-linked Alport syndrome: Three modes of inheritance exist for AS: X-linked Alport syndrome (XLAS) (mutations involving COL4A5 on the X chromosome), autosomal recessive Alport syndrome (ARAS) (mutations in COL4A3 or COL4A4 on chromosome 2), and autosomal dominant Alport syndrome (ADAS) (mutations in COL4A3 or COL4A4) (1, 3).