Hemoglobin S is a variant of hemoglobin, the protein in red blood cells responsible for transporting oxygen throughout the body. It differs from normal hemoglobin, known as hemoglobin A, by a single amino acid change in its structure. This alteration can lead to the distortion of red blood cells into a sickle shape, especially under low oxygen conditions. Individuals with two copies of the hemoglobin S gene have a condition called sickle cell disease, which can cause various health issues, including pain crises and increased risk of infections. Those with one copy of the gene are carriers, known as having sickle cell trait, and typically do not experience symptoms.