In the search for novel tubulin-targeting ligands, we employed the COMPARE algorithm. This approach is based on the idea that the anticancer activity profiles of drugs from the NCI database with a similar mechanism of action should be similar. The anticancer activity profiles are defined as the relative cytotoxicity patterns of drugs against > 60 cancer cell lines. We identified a list of the top 200 drugs whose anticancer activity profiles best correlated with those of colchicine (Table ). Of these 200 compounds, 106 compounds were close structural analogs of well-known colchicine site-binders, colchicine and podophyllotoxin; 72 were known inhibitors of tubulin polymerization, binding to the colchicine site or alternative sites. The mechanisms of action for the remaining 22 structures had not been previously characterized (Table , entries highlighted in bold). In order to annotate these 22 compounds, we used a spectral clustering algorithm, comparing them with 127 well-characterized reference compounds (see Methods for details).