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Kallmann's syndrome is a genetic condition characterised by hypogonadotrophic hypogonadism and anosmia; additional neurological and non-neurological symptoms may also occur depending on the specific mode of inheritance. Mode of inheritance can be X-linked (KAL-1), autosomal dominant (KAL-2) or autosomal recessive (KAL-3), although unrelated sporadic cases occur more frequently. The gene responsible for the X-linked form, namely KAL-1, and its encoded protein anosmin-1 have been identified for some time. Very recently the gene responsible for the autosomal dominant form was also identified. KAL-2 is caused by loss-of-function mutations in the gene encoding fibroblast growth-factor receptor-1 (FGFR1).

One of the genes involved in craniosynostosis syndromes is the fibroblast growth factor receptor 2 (FGFR2) gene, a tyrosine kinase receptor gene. Upon ligand binding the FGFR2 receptors dimerise, and this is followed by activation of the intracellular tyrosine kinase domains. This initiates a cascade of signals that influence cell division and differentiation. FGFR2 mutations have been found in the Apert, Crouzon and Pfeiffer craniosynostosis syndromes. Most mutations are gain of function mutations, inducing ligand-independent receptor activation or altered ligand binding. With the exception of Apert syndrome, there is no clear genotype-phenotype correlation. Many different mutations have been found in Pfeiffer and Crouzon syndrome, but all of the mutations occur in the same extracellular region of the receptor. Identical mutations have been found in Pfeiffer and Crouzon syndrome. So within one family, both Crouzon and Pfeiffer syndrome may occur. Mutations in other FGFR-genes have also been found in craniosynostosis syndromes.

The RET gene encodes a receptor tyrosine kinase involved in normal and neoplastic development of neural crest cell lineages. Activating RET mutations are present in patients with multiple endocrine neoplasia types 2A and 2B (MEN2A, 2B) and in familial medullary thyroid carcinoma (FMTC) patients, whereas inactivating RET mutations are found in patients with Hirschsprung (HSCR) disease. In particular for MEN2A and FMTC, the clinical management largely depends on the specific mutation found.

Hirschsprung's disease (HSCR) is a congenital disorder characterized by intestinal obstruction due to the absence of intramural ganglia along variable lengths of the colon. Occurrence among family members and recurrence risks among siblings are indications for involvement of genetic predispositions. Mutations have been discovered in five different susceptibility genes. One of the most important findings is the detection of specific mutations in the so-called RET gene, which can also be responsible for the multiple endocrine neoplasia type 2A (MEN2A) syndrome. HSCR patients with such specific mutations run an increased risk of developing MEN type 2A related tumours.