GNRH1 and Kallmann syndrome: The genetic defects underpinning CHH broadly fall into two principle groups, comprising (a) those causing neurodevelopmental defects of GnRH neuron migration frequently associated with non-reproductive defects, particularly anosmia/hyposmia from olfactory axon misrouting (i.e., Kallmann syndrome–KS), and (b) those causing pure neuroendocrine impairment of GnRH secretion or action (normosmic CHH).