Nevertheless, ∼10–15% of XLAS cases had a negative family history; those patients harbored COL4A5 de novo variants (Lemmink et al., 1997; Haas, 2009; Cervera-Acedo et al., 2017; Sun et al., 2021). The gene discussed is COL4A5; the disease is X-linked hydrocephalus with stenosis of the aqueduct of Sylvius.