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). The gene discussed is COL4A4; the disease is X-linked Alport syndrome.