Calnexin/CANX is a single-pass type I membrane protein which belongs to the calreticulin family. It consists of a large N-terminal calcium-binding lumenal domain, a single transmembrane helix and a short (90 residues), acidic cytoplasmic tail. The function of calnexin is to retain unfolded or unassembled N-linked glycoproteins in the endoplasmic reticulum. Calnexin is a calcium-binding protein that interacts briefly with newly synthesized glycoproteins in the endoplasmic reticulum. Calnexin may act in assisting protein assembly and/or in the retention within the ER of unassembled protein subunits. Calnexin seems to play a major role in the quality control apparatus of the ER by the retention of incorrectly folded proteins. Calnexin dwindles with aging and might contribute to a cytoprotection in an array of human age-related diseases.