PROK2 and Kallmann syndrome: A similar loss of GnRH neurons, together with hypofertility or infertility, is a hallmark of a number of murine models of Kallmann syndrome and normosmotic idiopathic hypogonadotropic hypogonadism, such as homozygous Fgf8 and Fgfr1 hypomorphs, in which GnRH neurons fail to emerge (Chung et al., 2008), Sema3e KO, in which a large fraction of GnRH neurons undergo apoptosis (Cariboni et al., 2015), Prok2 KO, and Prok2r KO, in which GnRH neurons show defective migration (Matsumoto et al., 2006; Pitteloud et al., 2007).