Sickle cell disease causes red blood cells to be crescent-moon-shaped. Sickle cell is caused by a DNA mutation that leads to a mutation in the hemoglobin protein. Surprisingly, the sequence of amino acids in the sickle cell hemoglobin has just one different amino acid compared to to the normal hemoglobin protein. (A Glu is changed to a Val). What type of DNA mutation is MOST likely the cause of this protein mutation