CDT1 and xeroderma pigmentosum: Upon 2 Gy IR irradiation, the majority of cells from patients with Hutchinson-Gilford (HGPS) syndrome, Werner syndrome (WS), and Rothmund Thomson syndrome (RTS) underwent Cdt1-switching, whereas the majority from patients with Bloom syndrome (BS), Cockayne syndrome (CS), and Xeroderma Pigmentosum (XP) as well as normal HCA2 did not (Fig. 4A).