The effective concentration of a growth inhibitor, which produces 50% of the maximum possible response for that inhibitor. GI50 is calculated from [(Ti-Tz)/(C-Tz)] x 100 = 50, where Tz is absorbance at time zero, Ti is absorbance in the presence of test drug, and C is absorbance in the control (untreated cells). As a measure of viable or proliferating cells, different researchers measure different parameters, namely, protein content (by sulforhodamine B staining followed by absorbance measurement), mitochondrial dehydrogenase activity (by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrasodium bromide, known as MTT staining followed by absorbance measurement), expression of proliferation associated antigens (by immunostaining for Ki-67), and ATP content (by using CellTiter-Glo reagent (Promega) followed by luminescence measurement).