AS (ORPHA 63) can be subdivided according to either the observed inheritance pattern or the underlying molecular defect into X-linked Alport syndrome (XLAS; ORPHA 88917), which is linked with pathogenic variants affecting the COL4A5 gene on the X chromosome, and autosomal recessive Alport syndrome (ARAS; ORPHA 88919), linked with biallelic pathogenic variants affecting COL4A3 or COL4A4, which are situated next to each other on chromosome 2 [5]. The gene discussed is COL4A4; the disease is X-linked hydrocephalus with stenosis of the aqueduct of Sylvius.