In this project we performed unbiased proteomics to uncover new ER-resident COPI cargo. We further filtered the results to identify proteins which are important for controlling cell growth. We identified TMEM263/C12orf23, a protein whose mutation is associated with organismal growth defects and dwarfism. Our findings demonstrate that TMEM263 is an ER-resident COPI cargo that regulates the early steps of lipid droplet formation.