Human cells contain 23 pairs of chromosomes. Most of an organism's chromosomes—generally all except for one pair—are called autosomes, which are the same in males and females. Humans have 22 pairs of autosomes. Many organisms also have a pair of sex chromosomes, which differ between males and females. In humans, a female has two identical sex chromosomes. A male has one sex chromosome that is like those of females, and one that is smaller and differently shaped.