Given the task definition and input, reply with output. In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

Notably, one of these entities is absent in the Leishmania mexicana GPI8 protein, and abnormally low numbers of pericytes occur if one is missing from PDGF. Binding of 14-3-3 proteins to GABAB ["Gabba-sub-bee"] receptors does not occur when two arginines are mutated to alanine in one of these entities, which also binds to the COPI complex. They are usually C-terminal signals, and a common mammalian one consists of the amino acids lysine, aspartate, gluatamate, and leucine, or KDEL. hnRNP C proteins contain one of them that allows them to override nuclear export signals. For 10 points, name these signals important in directing proteins to the correct membranes, most often used to prevent a protein's export from the endoplasmic reticulum. 

Science