PERK is a serine/threonine protein kinase that resides in the endoplasmic reticulum (ER) membrane and plays a critical role in the unfolded protein response (UPR). When misfolded proteins accumulate in the ER, PERK is activated to phosphorylate eIF2α (eukaryotic initiation factor 2α), which inhibits global protein synthesis to reduce ER stress while upregulating stress-related genes such as ATF4 to restore proteostasis or induce apoptosis.