ERCC4 and xeroderma pigmentosum: Using XPG- and XPF-deficient fibroblasts from XP patients and isogenic cell lines with added wild-type XPG and XPF, knockdown of AQR in cell lines supplemented with wild-type XPF or XPG resulted in phosphorylation of KAP1, while phosphorylation of KAP1 was reduced in the XPF- and XPG-deficient cell lines, which suggested roles for XPF and XPG in DSB formation and the processing of R-loops [31].