PTPA and cancer: When we published a review article entitled “Tumor promotion by inhibitors of protein phosphatases 1 and 2A: the okadaic acid class of compounds” (Fujiki and Suganuma 1993), we raised three hypotheses concerning how the okadaic acid pathway is related to human cancer: the first hypothesis was simply the exposure of the okadaic acid class compounds, the second was the involvement of endogenous protein inhibitors of PP1 and PP2A, and the third was that the effects of okadaic acid mimicked those of TNF-α and IL-1 in cells, as presented above.