In eukaryotes, most of the newly synthesized secretory proteins are transported from the ER to the Golgi apparatus. If these proteins have a particular 4-amino-acid retention sequence for the ERs lumen, KDEL, on their C-terminus, they are retained in the ERs lumen or are routed back to the ER's lumen (in instances where they escape) via interaction with the KDEL receptor in the Golgi apparatus. If the signal is KKXX, the retention mechanism to the ER will be similar but the protein will be transmembranal.