CYB5R4 is an NADH-cytochrome b5 reductase localized to the endoplasmic reticulum and cytosol that functions as an oxidoreductase enzyme, reducing substrates including cytochrome c, ferricyanide, and methemoglobin. The gene plays a critical role in protecting pancreatic beta-cells against oxidant stress by limiting reactive oxygen species accumulation. Beyond its classical reductase function, CYB5R4 sustains endothelial cell proliferation and ischemia-induced angiogenesis through an SCD-independent mechanism involving RRM2-dependent nucleotide balance 1. In mice, Cyb5r4 haploinsufficiency or endothelial-specific deletion impaired perfusion recovery following femoral artery ligation and reduced capillary expansion, positioning the CYB5R4-RRM2 axis as a potential therapeutic target for peripheral artery disease. CYB5R4 is upregulated in osteoporosis pathogenesis as part of a seven-gene diagnostic signature associated with endoplasmic reticulum stress 2, and variants show population-level signals of gene-environment interaction affecting glucose-induced insulin responses 3. Clinical relevance extends to podocyte cytoskeletal integrity and male reproductive health, where CYB5R4 downregulation correlates with infertility-associated dysfunction 4.