Point mutations are generally caused by a change in the single nucleotide, either by deletion, insertion or base change, in the DNA or RNA material; this is a change in the genetic material structure, the organism that is affected by the point mutation is known as appoint mutant, that is the organism that the genetic material changes. Point mutations sometime can cause a genetic disorder. For example, sickle cell anemia is a specific genetic disorder in human populations. It is caused by a point mutation of a single nucleotide at position 6 (GAG-GUG) in the -globin gene, and the glutamic acid being substituted by valine. This is a missense mutation. As we know, hemoglobin which is used by human's red blood cells to carry oxygen. And hemoglobin consists of four globin subunits (two 
-globin and two 
-globin). The -globin gene provides instruction for making -globin.Because of the point mutation, the hydrophobic side chain of the valine residue at position 6 of the beta chain in hemoglobin is able to associate with the hydrophobic patch, causing hemoglobin S molecules to aggregate and form fibrous precipitates. It will change the shape of red blood cell. Therefore, the ability of these red blood cells will decrease. Sickle-cell disease is associated with a number of acute and chronic health problems, such as severe infections, attacks of severe pain, stroke, and an increased risk of death.