The interesting findings on the elevated plasma activities of histaminase (diaminase or diamine oxidase), L-Dopa decarboxylase or calcitonin, in small-cell carcinoma of the lung, or C-cell hyperplasia of familial medullary thyroid carcinoma that were suggestive of embryonic origins that ‘small-cell lung carcinoma and medullary thyroid carcinoma, and further associate histaminases with neuronal crest tumors’ [122–125] are potentially important clues to better understand the origin and susceptibility of organs toward carcinogenesis. The gene discussed is AOC1; the disease is medullary thyroid gland carcinoma.