TAC3 and Gonadotropin deficiency: We studied the frequency of GNRHR, FGFR1, TAC3, and TACR3 mutations in nine adolescent and young adult females with clinical cues consistent with partial gonadotropin deficiency (stalled puberty, unexplained secondary amenorrhea), and describe phenotypic features and molecular genetic findings of monozygotic twin brothers with stalled puberty.