Additionally, we applied spectral clustering, implemented as described previously, to annotate the mechanism of action of the compounds identified by their correlation with colchicine or griseofulvin. For this purpose, each of the top 200 compounds correlating with colchicine or griseofulvin were sequentially pooled with a reference list of the 127 annotated NCI database drugs (Table ) and co-clustered with them. Each compound was either placed into one of the six groups with an established mechanism of action or assigned to the seventh group with an alternative anticancer activity pattern. The six reference groups included: (i) tubulin polymerization inhibitors, (ii) alkylating agents, (iii) topoisomerase I inhibitors, (iv) topoisomerase II inhibitors, (v) RNA and DNA antimetabolites and (vi) DNA antimetabolites.