A syndrome of defective gonadal development in phenotypic women with a karyotype of sex chromosome monosomy (45,X or 45,XO), associated with the loss of a sex chromosome X or Y. Patients generally are of short stature with undifferentiated (streak) gonads, sexual infantilism (HYPOGONADISM), webbing of the neck, cubitus valgus, elevated GONADOTROPINS and decreased ESTRADIOL level in blood. Studies of Turner Syndrome and its variants have contributed significantly to the understanding of SEX DIFFERENTIATION. NOONAN SYNDROME bears similarity to this disorder; however, it also occurs in males, has normal karyotype, and is inherited as an autosomal dominant.