COL4A2 and cerebral small vessel disease: COL4A1 and COL4A2 can encode different subunits of type IV collagen, a major structural component of the mucosa, and are associated with various diseases, including retinal artery tortuosity, cerebral small vessel disease with or without ocular abnormalities, cerebral small vessel disease and cerebral hemorrhage, which is closely associated with vascular disease (Volonghi et al., 2010; Kuo et al., 2012; Meuwissen et al., 2015).