COL4A1 and COL4A2 encode the proα1(IV) and proα2(IV) chains, respectively, that form heterotrimers (2 proα1(IV) chains and a single proα2(IV) chain) in the type IV collagen that forms the vascular basement membranes outside the kidney and between tissues of different origins (e.g., epidermis and dermis in skin). Pathogenic variants in COL4A1 and COL4A2, have been associated with intracerebral hemorrhage in neonates and children, intracerebral hemorrhage and cerebral small vessel disease in adulthood, intracranial aneurysms, retinal arteriolar tortuosity, aneurysms and muscle cramp (HANAC) syndrome, infantile hemiparesis, and porencephaly.