Finally, we note that a key difference between the studies that show a tumor-suppressive effect of MTH1 inhibition (including ours) [27,35,36,42] and those that do not [37,38,39] is that the former studies explicitly test effects of MTH1 inhibition in in vivo tumor formation models, whereas the latter solely utilize in vitro models (see Table 1). This evidence concerns the gene NUDT1 and neoplasm.