Gut granules are cell type-specific lysosome-related organelles found within the intestinal cells of Caenorhabditis elegans. To investigate the regulation of lysosome-related organelle size, we screened forC. elegansmutants with substantially enlarged gut granules, identifying alleles of the vacuolar-type H + -ATPase and uridine-5¢-monophosphate synthase (UMPS)-1.