Cysteine is a semi-essential amino acid containing a thiol group, crucial for protein synthesis, detoxification, and the formation of disulfide bonds that stabilize protein structures. In clinical practice, it plays a role in antioxidant defense and is a precursor to glutathione, a vital antioxidant in cellular protection. A key association is its involvement in cystinuria, a genetic disorder leading to kidney stone formation due to cystine accumulation.