CYB561 (cytochrome b561) is a transmembrane reductase that functions as a di-heme protein with dual roles in ascorbate homeostasis and iron metabolism 1. The protein uses ascorbate as an electron donor to reduce ferric iron (Fe³⁺) to ferrous iron (Fe²⁺) and regenerates monodehydroascorbate, supporting intravesicular enzyme activity and iron transport 2. CYB561 contains six transmembrane helices with two b-type heme centers coordinated by histidine residues 2. Beyond its classical biochemical function, CYB561 participates in neuropeptide activation through α-amidation in neuroendocrine cells 3. In cancer biology, CYB561 expression is significantly upregulated in breast cancer, castration-resistant prostate cancer, and non-small cell lung cancer tissues 43. In breast cancer, CYB561 promotes cell proliferation and invasion by activating the Akt/mTOR signaling pathway and regulating iron metabolism-associated proteins 5. High CYB561 expression correlates with poor prognosis, reduced recurrence-free survival, and is associated with macrophage M2 polarization in the tumor immune microenvironment 46. These findings suggest CYB561 serves as both a prognostic biomarker and potential therapeutic target in cancer.