CYB5R4 (cytochrome b5 reductase 4) is an NADH-dependent oxidoreductase that functions as a critical regulator of cellular redox homeostasis and endothelial proliferation. The enzyme reduces diverse substrates including cytochrome c and methemoglobin, with localization to the endoplasmic reticulum and cytoplasm 1. In endothelial cells, CYB5R4 sustains proliferation and ischemia-induced angiogenesis through an RRM2-dependent mechanism involving nucleotide pool balance rather than through its canonical stearoyl-CoA desaturase-promoting function 1. CYB5R4 is implicated in protection against oxidative stress and appears essential for podocyte cytoskeletal integrity, with silencing causing significant downregulation of synaptopodin 2. Clinically, CYB5R4 emerges as a key endoplasmic reticulum stress-related gene in osteoporosis pathogenesis, upregulated in disease models and included in a seven-gene diagnostic model with >0.9 AUC for early osteoporosis detection 3. CYB5R4 variants also show associations with energy-balance regulation and glucose-induced hormonal responses in human populations 4. Additionally, de novo truncation variants in CYB5R4 have been identified in neurodevelopmental disorders 5, and under-expression of CYB5R4 occurs in infertile males with defective spermatogenesis 6, suggesting roles in cellular development and gametogenesis. These findings position CYB5R4 as a multifunctional redox enzyme with therapeutic potential in vascular and metabolic diseases.