Mutations in the extracellular cysteine-rich region of the tyrosine kinase receptor, usually codons 609, 611, 618, 620, 630, or 634, cause MEN2A, an autosomal dominant, highly penetrant syndrome predisposing to early-onset MTC, bilateral phaeochromocytoma and hyperparathyroidism [16]. This evidence concerns the gene RET and hyperparathyroidism.