Maintains Cellular Health* With Selenium N-acetyl cysteine (NAC) is a stable form of the non-essential amino acid cysteine. It is a sulfur-containing amino acid that acts as a stabilizer for the formation of protein structures and is also necessary for the formation of glutathione.* Selenium is an essential trace mineral that facilitates the production of several important enzymes.* As a family-owned company since 1968 — NOW produces high-quality natural products at prices everyone loves. We are a respected leader in the natural products industry with a mission to provide value in products and services that empower people to lead healthier lives.