Despite the rapid growth of genetics in the last quarter of a century, there is little in the modern medical textbooks to indicate the applications of genetics to medicine. Statements that are obviously incorrect with respect to the inheritance of disease still find a place in modern textbooks. To some extent this is true of the rare condition of xeroderma pigmentosum. Although it was early recognized that this disease is in part dependent on a constitutional factor, it is not generally conceded that this factor is inherited. Thus, one still finds the statement that xeroderma pigmentosum is not hereditary but "familial." In the 1927 edition of his textbook on diseases of the skin Sequeira stated: "It does not appear to be hereditary, the fact that the patients rarely survive puberty probably explaining this." This mistaken idea is due, as will shortly be seen, to a misunderstanding of