Two siblings have been reported whose clinical manifestations (cutaneous photosensitivity and central nervous system dysfunction) are strongly reminiscent of the DeSanctis-Cacchione syndrome (DCS) variant of xeroderma pigmentosum (XP), a severe form of XP. Fibroblasts from the siblings showed UV sensitivity, a failure of recovery of RNA synthesis (RRS) after UV-irradiation, and a normal level of unscheduled DNA synthesis (UDS), which were, unexpectedly, the biochemical characteristics usually associated with Cockayne syndrome (CS). However, no complementation group assignment in these cells has yet been performed. We here report that these patients can be assigned to CS complementation group B (CSB) by cell fusion complementation analysis. To our knowledge, these are the first patients with defects in the CSB gene to be associated with an XP phenotype. The results imply that the gene product from the CSB gene must interact with the gene products involved in excision repair and associated with XP.