Heredity depends on the intrinsic factors, the genes, and on extrachromosomal influences. Mendelian inheritance is through the chromosomes, and the various traits are carried by the hypothetic biochemical units called genes. The factor of heredity in malignant tumors is of minor importance, with the exception of glioma and neurofibromatosis (von Recklinghausen's disease), in which the hereditary influence is generally recognized and accepted. In addition to these two types of tumors, Lehmann1 stated that polyposis of the rectum is irregularly dominant, a precancerous condition, with a possibility of development into cancer in 75 per cent of cases, and that xeroderma pigmentosum has a recessive inheritance, with consanguinity in 25 per cent of cases. Xeroderma pigmentosum, with an inborn excess of porphyrins, like polyposis of the rectum, represents a precancerous state, a hereditary hypersensitiveness of the skin to sunlight, especially the ultraviolet rays, resulting in development of carcinoma of the skin,