For many biochemical compounds in the cell, the endoplasmic reticulum is only a place of temporary residence where they are manufactured and processed before being transported to other locations. Some proteins, however, are designed specifically to remain permanently in the ER lumen, where they play an important role in the function of the organelle. The ER knows not to export these resident proteins because they contain a specific sequence of amino acids that together act as a specialized retention signal. One particularly important ER resident protein is the BiP chaperone protein that functions in the identification of improperly processed proteins and prevents them from being transported to other regions of the cell. The proximity of the endoplasmic reticulum to the mitochondrial network was visualized in the digital video sequences presented in this section with mEmerald fused to an ER targeting signal and mCherry fused to a mitochondrial targeting signal.