Research d irected toward und erstand ing genetically d etermined d isord ers in man is of consid erable interest in the biomed ical sciences. these d isord ers is a group of Mend elian single-gene traits ty pified by xerod erma pigmentosum (XP), sy nd rome), Arlett & Lehmann 1978, affl icted with any one of these autosomal recessive sy nd romes are at high risk of cancer, and increased sensitivity to specific extrinsic agents known to d amage DNA. in one or more of the enzy matic processes med iating the repair of d amaged cellular DNA. so much in their contribution to public ill health, but rather in the fact that their very existence attests to the fund amental importance of functional DNA repair processes to the well-being of man and in the suggestion that faulty repair may be causally related to malignant transformation. Cells d erived from aff ected patients have become valuable research tools. of mutants wi th whi ch to clarify th e di fferent enzy mati c repai r processes operating in human cells on DNA d amaged by numerous carcinogens and other toxic environmental agents (Cleaver & Bootsma 1975, al 1977,