The nitroreductase expression in COS-7 cells is improved by codon usage optimization. (A) At the indicated time points cleared cellular lysates (20 μg) of pGNTR and pGNTRo-transfected cells were analyzed by immunoblotting using anti-GFP and anti-actin (loading control) antibodies. (B) Densitometric quantification of GNTR protein levels normalized to actin. Combined data of two independent experiments are represented as means ± SEM. *: p < 0.05; n = 2. (C, D) Consistently, the amino- (C) and carboxyterminally c-myc-tagged (D) ntro showed similarly improved expression levels as compared with the corresponding ntr constructs.