Cysteine is a semi-essential amino acid that plays a crucial role in protein synthesis and metabolism. It contains a sulfur atom, which allows it to form disulfide bonds, contributing to the stability and structure of proteins. Cysteine can be synthesized in the body from another amino acid called methionine, making it semi-essential, as it may need to be obtained from the diet under certain conditions. This amino acid is found in various foods, including meat, fish, eggs, and dairy products. It is also present in some plant-based sources like garlic and cruciferous vegetables. Cysteine is important for the production of glutathione, a powerful antioxidant that helps protect cells from damage.