0 or 1 of previous expression; also forces minimal matching when an expression might match several strings within a search string. \. Preceding one of the above, it makes it a literal instead of a special character. Preceding a special matching character, see below.
sickle cell disease. Sickle cell anemia, a common form of sickle cell disease, is caused by a particular mutation in the HBB gene. This mutation results in the production of an abnormal version of beta-globin called hemoglobin S or HbS. In this condition, hemoglobin S replaces both beta-globin subunits in hemoglobin.
One example of a point mutation is Sickle Cell Anemia. A point mutation caused a single nitrogen base in a codon for one amino acid in the protein called glutamic acid to instead code for the amino acid valine. This single small change causes a normally round red blood cell to instead be sickle shaped.