Assembly of the phospholipid bilayer of cellular membranes is a fundamental aspect of cell growth and proliferation. Phospholipids are concomitantly synthesized and inserted at the cytoplasmic surface of the endoplasmic reticulum. Following this asymmetric assembly, transmembrane movement to the lumenal leaflet of the endoplasmic reticulum must occur in order to ensure coordinated growth of the bilayer. For phosphatidylcholine, the predominant phospholipid of eukaryotic membranes, this latter process appears to be facilitated by a specific transport protein.