Hereditary medullary thyroid carcinoma (MTC) is characterized by germline RET mutations [38] and in 40–60% of sporadic MTC somatic RET mutations are present [39, 40], which cause constitutive activation of the RET tyrosinkinase and in consequence MAPK/ERK and PI3K pathway activation. The gene discussed is PIK3CA; the disease is familial medullary thyroid carcinoma.