What missing protein causes sickle cell?

Sickle cell disease is caused by a mutation in the gene that encodes for hemoglobin, specifically the beta-globin subunit of hemoglobin A. This mutation leads to the production of hemoglobin S (HbS), which causes red blood cells to become rigid and sickle-shaped under low oxygen conditions. The primary issue is not the absence of a protein but rather the presence of an abnormal form of hemoglobin that disrupts normal red blood cell function.