Phosphatidylcholine transfer protein (PC-TP) is a highly specific soluble lipid binding protein that transfers phosphatidylcholine between membranes in vitro. PC-TP is a member of the steroidogenic acute regulatory protein-related transfer (START) domain superfamily. Although its biochemical properties and structure are well characterized, the functions of PC-TP in vivo remain incompletely understood. Studies of mice with homozygous disruption of the Pctp gene have largely refuted the hypothesis that this protein participates in the hepatocellular selection and transport of biliary phospholipids, in the production of lung surfactant, in leukotriene biosynthesis and in cellular phosphatidylcholine metabolism. Nevertheless, Pctp(-/-) mice exhibit interesting defects in lipid homeostasis, the understanding of which should elucidate the biological functions of PC-TP.