Turner syndrome or Ullrich-Turner syndrome encompasses several chromosomal abnormalities, of which monosomy X is the most common. It occurs in 1 out of every 2500 female births. Instead of the normal XX sex chromosomes for a female, only one X chromosome is present and fully functional. A normal female karyotype is labeled 46,XX
; individuals with Turner syndrome are 45,X
(also labeled 45,X0
or less commonly X(,)
) though other genetic variants occur. In Turner syndrome, female sexual characteristics are present but generally underdeveloped.