Glutamyl cysteine synthetase (GCS) is the key enzyme in the synthesis of glutathione (GSH), consisting of a catalytic and a regulatory subunit. GCS is located in the cytoplasm and is widely distributed across various human tissues, particularly in the liver and red blood cells, where there is a high demand for antioxidant protection. It catalyzes the formation of glutamyl cysteine from glutamine and cysteine, which is the first step in GSH synthesis. GSH is an important antioxidant involved in cellular defense, detoxification, and signal transduction. The specific expression of the GCS gene is regulated by various cellular signals, and mutations or dysregulation in its expression are associated with diseases related to oxidative stress, liver diseases, and immune dysfunction.