Furthermore, non-syndromic HD is often associated with loss-of-function mutations in RET, including missense, nonsense, and frameshift mutations that impair RET signaling, leading to aganglionosis in the distal colon without predisposing individuals to MTC or PHEO. Haploinsufficiency is the most likely mechanism, but it is not sufficient to explain its coexistence [2,5,6,9-10]. This evidence concerns the gene RET and Huntington disease.