Complementation test. The classical complementation assay for nucleotide excision repair (NER) defects is based on the analysis of unscheduled DNA synthesis (UDS) in heterodikaryons obtained following fusion of primary dermal fibroblasts of the patient under study with cells representative of each of the various XP groups. To easily identify the fusion products, the two cell strains used as partners in the fusion are labelled with beads of different size. The two cell strains are classified in the same complementation group if the heterodikaryons, identified as binuclear cells containing beads of different sizes, fail to recover normal UDS levels and remain at the low levels seen in the mononuclear cells (right panel). Conversely, the restoration of normal UDS levels in the heterodikaryons (left panel) indicates that the cell strains used as partners in the fusion have genetically different defects (courtesy of Tiziana Nardo, IGM CNR Pavia).