Most (>95%) SVD is nonheritable, or sporadic, although over 50 genetic loci associated with the risk of sporadic SVD have emerged from genome-wide association studies in recent years.13–15 Heritable, monogenic forms of SVD include cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL; due to mutations in NOTCH3), CARASIL (HTRA1), and collagen-IV (COL4A1/COL4A2).14 Relative to the prevalent, sporadic SVD, these are rare, less strongly associated with hypertension and detected clinically in younger people, usually with more severe disease. Here, COL4A1 is linked to snowflake vitreoretinal degeneration.