Turner synrome or Ullrich-Turner syndrome encompasses several conditions, of which monosomy X is the most common. It occurs in about 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; in rarer cases a second X chromosome is present but abnormal, while others with the condition have some cells with a second X and other cells without it (mosaicism). A normal female karyotype is labeled 46,XX; individuals with Turner syndrome are 45,X. In Turner syndrome, female sexual characteristics are present but generally underdeveloped.