*4.3. UVB Irradiation*

After pretreatment and incubation, the culture medium was removed, and cells were covered with a thin layer of DPBS. Cells were irradiated with 20 mJ/cm<sup>2</sup> UVB, using two UVB broadband tubes (TL-20W/12 RS; Philips, Eindhoven, The Netherlands). For the *HPRT* gene mutation assay, the UVB dose was reduced to 10 mJ/cm<sup>2</sup> , as it was found to be the most mutagenic for CHO cells (Figure 2). The proper dosage of UVB was determined by a UVX Digital Radiometer (UVP Inc., San Gabriel, CA, USA). After irradiation, the DPBS was replaced by DMEM supplemented as described above.
