We investigated whether a heterozygous mutation that we newly identified in HTRA1 (high‐temperature requirement serine protease A1 gene) in a pedigree with autosomal dominant hereditary cerebral small vessel disease (SVD) reduces the function of HTRA1 and affects the transforming growth factor‐β1 (TGF‐β1)/Smad signaling. The gene discussed is TGFB1; the disease is snowflake vitreoretinal degeneration.