Consistent with our findings with the depleted RKO cells, primary fibroblasts isolated from patients suffering from RS-SCID and deficient for ARTEMIS (Guetel cells) [16] or from a severe form of Xeroderma Pigmentosum and deficient for XPF [17] when treated with HU had less p53BP1 induction than had wild-type primary fibroblasts isolated from healthy donor (Fig 3D). The gene discussed is ERCC4; the disease is xeroderma pigmentosum.