scue of hemizygous XpdTTD/KO survival by Xpd†XPCS and Xpd†XP alleles is illustrated by arrows marked A and B, respectively. UV survival of homozygous XpdXPCS/XPCS cells (asterisk) from the normally expressed viable allele (XpdXPCS) is depicted by a dotted line. Survival curves represent an average of four independent experiments; 1–2 cell lines per genotype were included in each experiment. Error bars indicate SEM