Calmegin, also known as CLGN, is a protein which in humans is encoded by the CLGN gene. Calmegin is also present in other animals including mice.Calmegin is a testis-specific endoplasmic reticulum chaperone protein. CLGN may play a role in spermatogeneisis and infertility.

The proper folding of newly synthesized membrane proteins in the endoplasmic reticulum (ER) is required for the formation of functional mature proteins. Calmegin is similar to calnexin, a ubiquitous ER chaperone that plays a major role in quality control by retaining incompletely folded or misfolded proteins. In contrast to other known chaperones, such as heat shock proteins and calreticulin, calnexin is an integral membrane protein .