HTRA1 and snowflake vitreoretinal degeneration: There are monogenic forms of SVD that include mutations in the human NOTCH3 gene causing cerebral dominant arteriopathy with cerebral infarcts and leukoencephalopathy (CADASIL) [4], mutations in the human COL4A1 OR COL4A2 gene causing haemorrhagic SVD [5, 6] and mutations in the gene encoding the serine protease HTRA1, causing cerebral recessive arteriopathy with subcortical infarctions and leukoencephalopathy (CARASIL) [7].