Among compounds 1c, 2c, 2d, 3b that had shown weak effects in vitro and no effects on division of cancer cells , only 1c displayed a tubulin-binding signal in the C-MST assay. The measurements clarified the binding sites of the active compounds, indicating that they all bind to the colchicine pocket of tubulin, consistent with our previous modeling predictions. We also screened the compounds 1g-p, which were not characterized previously, and observed no tubulin binding signals (Fig. ).