The DDB2 proteo-probe assay detects GG-NER defects in XP cell lines, but not CS or TTD cell lines. A. Cells harvested before, 5 minutes after, or 120 minutes after UV exposure are treated with the purified DDB2 proteo-probe. In all cell lines, DDB2 foci are absent prior to UV exposure but are present 5 minutes after UV exposure. In cells with intact GG-NER, DDB2 foci resolve by 120 minutes, indicating repair, whereas foci persist in cells lacking NER. B. The DDB2 assay was performed on a panel of fibroblast cell lines established from patients with XP, CS, or TTD. Mean GG-NER deficiency is calculated by dividing the DDB2 proteo-probe signal present 2 hours following UV exposure by the signal present 5 minutes after exposure. XP cell lines (red) are GG-NER deficient while CS, TTD, and XP variant cell lines have GG-NER capacity similar to a control fibroblast cell line (BJ1).