IHH associated with impaired olfactory function (Kallmann syndrome) may be caused by mutations of the X-chromosomal KAL1 (encoding anosmin) or the fibroblast growth factor receptor 1 genes (FGFR1), both leading to agenesis of olfactory and GnRH-secreting neurons.
To date, the mutation spectra of non-syndromic form of familial and sporadic tooth agenesis in humans have revealed defects in various such genes that encode transcription factors, MSX1 and PAX9 or genes that code for a protein involved in canonical Wnt signaling (AXIN2), and a transmembrane receptor of fibroblast growth factors (FGFR1).