Activity of rtTAG19F E37L variants with all possible amino acids at position 56. (A) The activity of rtTA was measured in HeLa X1/6 cells, see Fig. 2 for details. All variants contain the G19F, E37L, F86Y and A209T mutations in combination with different amino acids at position 56. The wild-type rtTA (wt) with only the F86Y and A209T mutations was included as a control, of which the activity at 1000 ng/ml dox was arbitrarily set at 100%. Average values of two transfections are shown with the error bars indicating the standard deviations. (B) Codon table of rtTAG19F E37L variants with all possible amino acids at position 56. The corresponding codons of inactive variants are marked in black, dox-dependent variants are in green, and variants that are active without dox are in red. See the text for details.