Phospholipid (PL) and triacylglycerol (TG) associate with the apo B-48 polypeptide as it is translated. This process is mediated by MTP (microsomal triacylglycerol transfer protein) in the form of a MTP:PDI (protein disulfide isomerase) heterodimer. MTP in vitro binds small amounts of PL and TG (annotated here as as one molecule of each) and efficiently transfers the bound lipid between membranes (Atzel and Wetterau 1994). In vivo, MTP:PDI directly interacts with apoB-48 polypeptide (Wu et al. 1996), and is thought to transfer lipid from the endoplasmic reticulum membrane to nascent apoB-48. While some of the molecular details of MTP function remain unclear, this function is clearly essential in vivo, as patients who lack MTP cannot produce chylomicrons (e.g., Wetterau et al. 1992; Narcisi et al. 1995).