In the consideration of human tumors and their inheritance, it is not always possible to establish a clear differentiation between the factors of chance and heredity in such common cancers as those of mammary, uterine, pulmonary, and gastric origin. Genetic factors influencing the development of neoplastic diseases seem to operate under 2 conditions: (a) when a dominant gene is causal as in the cases of intestinal polyposis, chondromatosis, neurofibromatosis, and retinoblastoma; (b) when genetic factors are present but not solely accountable in themselves for the onset of the cancer. In the latter category, C. M. Woolf1 lists cancer of the prostate, which disease he found to be 3 times more frequent in male relatives of prostatic cancer patients than in the rest of the population. Malignant melanoma occurring in more than 1 member of a family has led to the suggestion of a possible hereditary factor because of the