CIAO3 (cytosolic iron-sulfur assembly component 3) is a core component of the cytosolic iron-sulfur (Fe-S) protein assembly (CIA) complex that mediates incorporation of [4Fe-4S] clusters into extramitochondrial Fe/S proteins 1. CIAO3 forms a stable ternary complex with CIA2A and CIAO1, with its C-terminal [4Fe-4S] cluster playing a structural role in this interaction 1. The protein negatively regulates HIF-1α expression through the Fe-S cluster assembly pathway 2. Mechanistically, CIAO3 deficiency disrupts CIA complex assembly, impairing Fe-S cluster transfer to client proteins. This causes functional conversion of cytosolic aconitase to iron regulatory protein 1, leading to iron overload, oxidative stress, and suppression of anti-ferroptotic defenses 3. CIAO3 knockout in mice causes mid-gestational embryonic lethality due to impaired vascular development and endothelial dysfunction 3. Clinically, CIAO3 mutations cause diffuse pulmonary arteriovenous malformations (dPAVMs) associated with VEGF overexpression and iron accumulation 4. CIAO3 deficiency in endothelial cells triggers acute lung injury through immune activation, with TNF-α playing a central role and macrophages as key effector cells 5. Polymorphisms in NARFL (the CIAO3 ortholog) confer susceptibility to vascular and neurodegenerative disorders 3, positioning CIAO3 as critical for maintaining endothelial health and vascular homeostasis.