Seipin is required for converting nascent to mature lipid droplets.

How proteins control the biogenesis of cellular lipid droplets (LDs) is poorly understood. Using and human cells, we show here that seipin, an ER protein implicated in LD biology, mediates a discrete step in LD formation-the conversion of small, nascent LDs to larger, mature LDs. Seipin forms discrete and dynamic foci in the ER that interact with nascent LDs to enable their growth.