STARD4 is a cytosolic sterol transfer protein that plays a critical role in intracellular cholesterol transport and homeostasis 1. The protein consists almost entirely of a START domain and belongs to a subfamily with STARD5 and STARD6, sharing approximately 30% amino acid identity with these homologues 2. STARD4 functions as a non-vesicular cholesterol transporter, rapidly equilibrating sterol between membranes, particularly those with anionic lipid headgroups 3. The protein localizes to the cytoplasm and endoplasmic reticulum (ER), where it co-localizes with ACAT-1 and increases ACAT activity 2-fold by delivering cholesterol for esterification 4. STARD4 expression is negatively regulated by cholesterol via SREBP-2 through a functional sterol regulatory element in its promoter 5 and is induced during early ER stress through ATF6 and an ERSE-like element 6. Stable reduction of STARD4 results in increased total cholesterol, slower cholesterol trafficking, altered membrane fluidity, and compensatory changes in lipid homeostasis 1. In disease contexts, STARD4 acts as a tumor suppressor in prostate cancer by regulating cholesterol metabolism and AR signaling, with reduced expression correlating with adverse prognosis 7.