COL4A5 and X-linked hydrocephalus with stenosis of the aqueduct of Sylvius: Genotype–phenotype correlation has been demonstrated with XLAS; in particular, more severe or early onset renal phenotypes are found in male carriers of COL4A5 large deletions or truncating variants resulting in lower or absent levels of functional protein, with 90% risk of ESRD by age 30 [25,26].