: Barr body is the darkly stained sex chromatin mass which remain attached to the nuclear membrane of normal female somatic cells. It was seen by Murray Barr in 1949, for the first time in the skin cells of cats, mucous cells of mouth cavity, nerve cells and nucleus of neutrophils in blood. It is found in females only. According to the Lyon hypothesis, one of the two X chromosomes in each somatic cell of the female is genetically inactivated. The barr body represents the inactivated X chromosome.