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.
Sickle cell disease occurs from genetic changes which causes a portion of the hemoglobin molecules to be abnormal: 1  Hemoglobin A (HbA) . 2  HbA is the hemoglobin molecule found in normal red blood cells during childhood and adulthood. Hemoglobin A (HbA) . 2  HbA is the hemoglobin molecule found in normal red blood cells during childhood and adulthood. 3  People without sickle cell anemia have primarily this type of hemoglobin in their blood cells.