Defects in olfactory development and GnRH neuron migration are thought to be the primary cause of the congenital disorder known as Kallmann syndrome (KS); this notion is supported by various mutant mouse phenotypes (see below) and by the observation of a single human foetus affected by KS (Schwanzel-Fukuda and Pfaff, 2002). The gene discussed is GNRH1; the disease is Kallmann syndrome.