TMCC1 is an endoplasmic reticulum (ER) membrane protein that functions as a regulator of ER-associated endosome fission 1. The protein localizes to contact sites between the ER and endosomes, where it promotes recruitment of the ER to endosome tubules to facilitate membrane fission 1. TMCC1 contains two adjacent transmembrane domains near its C-terminus and coiled-coil domains, with its N-terminal and cytosolic C-terminal regions extending into the cytoplasm 2. These cytosolic regions form homo- and hetero-oligomers with other TMCC family proteins 2. Functionally, endosome membrane fission mediated by TMCC1 is essential for cargo sorting, separating regions destined for lysosomal degradation from carriers recycled to the plasma membrane 1. When TMCC1 is depleted, endosome morphology remains normal and buds form, but ER-associated bud fission and subsequent cargo sorting to the Golgi are impaired 1. TMCC1 recruitment requires Coronin 1C, an endosome-localized actin regulator, suggesting tightly regulated timing of ER recruitment after proper cargo sequestration 1. Beyond its canonical ER function, TMCC1 has emerged as a biomarker in disease contexts; dysregulation of TMCC1-AS1 (an antisense lncRNA) associates with hepatocellular carcinoma prognosis and immune responses 345, and TMCC1 transcript expression correlates with tuberculosis disease status 6.