Members of the LDL receptor gene family mediate cellular uptake of various extracellular ligands, including lipoprotein particles. Ligand interactions with these receptors can be antagonized by a 39 kDa receptor‐associated protein. Recent biochemical, cellular, and genetic studies have shown that receptor‐associated protein is a molecular chaperone/escort protein for LDL receptor‐related protein, a member of the LDL receptor gene family that binds multiple ligands. These studies indicate that receptor‐associated protein interacts with LDL receptor‐related protein at multiple sites and assists the proper folding and disulfide bond formation of LDL receptor‐related protein within the endoplasmic reticulum. Following the completion of folding, receptor‐associated protein remains associated with the receptor during its subsequent trafficking along the early secretory pathway, thereby preventing premature ligand interaction with the receptor. The ability of receptor‐associated protein to universally inhibit ligand interactions with members of the LDL receptor gene family underscores the use of this protein as a tool in the study of ligand–receptor interactions. Curr Opin Lipidol 9: 149–155.© 1998 Rapid Science Ltd