The vascular BM contains a collagen IV network of α1α1α2(IV) proteins consisting of two α1(IV) chains and one α2(IV) chain, encoded by the genes COL4A1 and COL4A2. 5Mutations in COL4A1/COL4A2 cause early-onset CSVD with ICH, eye, muscle and kidney defects as part of the multi-systemic COL4A1 syndrome (referred to as Gould Syndrome).5 Here, COL4A2 is linked to Down syndrome.