RAC1 and colonic neoplasm: On the other hand, under FBS stimulation in human colon cancer cell lines, the activity of endogenous Cdc42 was much lower than that of Rac1 (Supplementary Fig. S5a), and the Rac1 inhibitor NSC23766 clearly reduced membrane staining in FFPE cell blocks with the GST-PAK probe (Supplementary Fig. S5b), suggesting that this system can in principle detect both Rac and Cdc42, but here is likely to reflect the activation of endogenous Rac1.