COL4A5 and X-linked hydrocephalus with stenosis of the aqueduct of Sylvius: XLAS is caused by a pathogenic variant in COL4A5 (OMIM*303630) gene encoding type IV collagen (Col-IV) α5 chain, while ADAS and ARAS are consequences of a variant in COL4A3 (OMIM*120070) and COL4A4 (OMIM*120131) genes that encode Col-IV α3 and α4 chains, respectively.