Cell viability assays

Triplicates of 5 × 10³ cells were seeded one day before the drug treatment in a 96-well flat-bottomed plate in a total volume of 100 μl per well in complete medium. 50 μl of serial diluted drug CDK9 inhibitor AT7519 (Selleck Chemicals, Houston, TX, USA) or BN-09-LN53 (Novartis, Basel, Switzerland) was added to each well. Triplicate blank wells were set up by adding the same volume of complete medium and diluted drug solvent DMSO. At the end of treatment, Resazurin was added to each well. After 2 hours’ incubation, absorbance was measured at ex544/em590 nM using FLUOstar Omega plate reader (BMG Labtech, Offenburg, Germany). The average absorbance values were calculated from triplicates and the average from blank wells was subtracted from all other absorbance values. Relative viability rates were calculated by normalizing to the rate of DMSO-treated cells and the error bars indicate standard deviation (SD).