Codon optimization increases the cytotoxicity of the NTR/CB1954 system in transiently transfected cells. (A) COS-7 cells, transfected with pGNTR, pGNTRo or pEGFP-C1 (control) were cultivated in presence of increasing CB1954 concentrations. The survival of transfected cells (green) was analyzed by fluorescence microscopy 24 hours after CB1954 treatment. The DNA was stained with DAPI (blue). Scale bar = 100 μm. (B) The prodrug-mediated cell death was quantified by counting GNTR-expressing cells (green) 48 hours after treatment with CB1954. Cells expressing gntro were nearly fully depleted at concentrations below 25 μM, whereas a similar effect in gntr-expressing cells was observed only at CB1954 concentrations higher than 100 μM. Shown is one representative experiment out of two repetitions. *: p < 0.05; n = 3.