Effect of human NQO1 expression on the toxicity of menadione to CHO cells. Cells were plated out at the appropriate cell densities and exposed to menadione for 2 hr, followed by rinsing twice with Puck’s Saline G and the addition of complete growth medium. Colonies were fixed and stained after 10 days, and surviving fractions were determined. The D50 for CHO cells to menadione was ~2 μg/ml and to human NQO1-expressing CHO cells was ~2.8 μg/ml. Only the two CHO clones expressing the highest levels of human NQO1 were tested for differences in sensitivity to menadione. ●, CHO: ▲, DTD812; ◆, DTD818.