FGFR1 and hypogonadotropic hypogonadism: Similarly, FGFR1 mutations demonstrate domain-specific effects: variants in the extracellular Ig-like domains cause Pfeiffer syndrome (OMIM: 101600), characterized by craniosynostosis, broad thumbs, and great toes, while mutations in the kinase domain are associated with Kallmann syndrome (OMIM: 147950), distinguished by hypogonadotropic hypogonadism and anosmia40–42.