<p>Proteins resident in the lumen of the endoplasmic reticulum (ER) contain a C-terminal tetrapeptide, commonly known as Lys-Asp-Glu-Leu (KDEL) in mammals and His-Asp-Glu-Leu (HDEL) in yeast (<taxon tax_id="4932">Saccharomyces cerevisiae</taxon>) that acts as a signal for their retrieval from subsequent compartments of the secretory pathway. The receptor for this signal is a ~26 kDa Golgi membrane protein, initially identified as the ERD2 gene product in S. cerevisiae. The receptor molecule, known variously as the ER lumen protein retaining receptor or the 'KDEL receptor', is believed to cycle between the cis side of the Golgi apparatus and the ER. It has also been characterised in a number of other species, including plants, Plasmodium, Drosophila and mammals. In mammals, 2 highly related forms of the receptor are known. </p> <p> The KDEL receptor is a highly hydrophobic protein of 220 residues; its sequence exhibits 7 hydrophobic regions, all of which have been suggested to traverse the membrane [<cite idref="PUB00001233"/>]. More recently, however, it has been suggested that only 6 of these regions are transmembrane (TM), resulting in both N- and C-termini on the cytoplasmic side of the membrane.</p>