The xeroderma pigmentosum complementation group D is defined by more than 30 unrelated individuals of whom less than half show major abnormalities of the central nervous system, once considered to be the hallmark of the group. Fibroblasts from the great majority of these individuals show very considerable sensitivity to UV light in vitro despite the fact that the cells carry out what appears to be substantial excision repair, as judged from repair synthesis and incision activity. This article reviews the XPD group and the defects in cellular DNA repair and examines the lack of correlation between repair and the appearance of neurological abnormalities. The article also discusses the recent awareness that at least some members of two other inherited conditions, trichothiodystrophy and Cockayne's Syndrome, carry mutations in the XPD gene.