COL4A3 and X-linked hydrocephalus with stenosis of the aqueduct of Sylvius: To date, the association between Col4 defects and PKD has been statistically demonstrated only in carriers of variants in the COL4A3 and COL4A4 genes, yet it was observed that a significant percentage of XLAS cases had a radiological cisternal phenotype similar to that of ADAS patients [17].