Three distinct genetic forms of the disorder exist: (1) X-linked Alport syndrome, linked to mutations in the COL4A5 gene, (2) autosomal recessive Alport syndrome with mutations in both alleles of COL4A3 or COL4A4 genes, and (3) autosomal dominant Alport syndrome also associated with heterozygous mutations in the COL4A3 or COL4A4 genes. The gene discussed is COL4A4; the disease is Alport syndrome.