To test whether overexpression of cancer-derived mutants of IDH1 or IDH2 could reduce total cellular NADP+-IDH activity at other concentrations of isocitrate at which mutant:WT heterodimers have lowered activity, we also assayed the total cellular NADP+-IDH activity of the 293T cells expressing IDH1-R132H or IDH2-R172K described above over a range of 0 to 80 μM isocitrate. Here, IDH2 is linked to cancer.