Some proteins exiting the ER are returned to the ER by COPI coated vesicles.  These proteins are identified by the presence of specific signal sequences that interact with the COPI vesicles or associate with specific receptors.
					Example of retrieved protein: ER chaperones like BiP that are mistakenly transported.
	This example describes the situation of BiP.  BiP has the signal sequence, KDEL.  When BiP escapes the ER, it associates with the KDEL receptor.  The slightly acid environment of the vtc and Golgi favor this association.  When the returning vesicle fuses with the ER, the neutral pH of the ER causes BiP to dissociate from the receptor.