UDS assay. UV-induced DNA repair synthesis (unscheduled DNA synthesis, UDS) on autoradiographic preparations of primary dermal fibroblasts irradiated with UV light and incubated with 3H-thymidine, a DNA radioactive precursor. The labelling pattern in G1 and G2 nuclei reflects the amount of precursor incorporated during repair synthesis. Therefore, the number of grains is a direct and quantitative measurement of the ability of the cell to perform excision repair. Compared to normal cells (left panel), cells from a XP patient (right panel) show fewer grains and, thus, a reduced ability to perform UDS (courtesy of Tiziana Nardo, IGM CNR Pavia).