Drugs affecting the cell cycle provide insights into mechanisms underlying cancer and suggest strategies for ablating uncontrolled growth. Essential to an understanding of the activity of such compounds is the identification of the set of proteins affected, either directly or indirectly, by the drug. The combination of novel technologies for stable isotope protein tagging, chromatographic separation, tandem mass spectrometry, and data processing is an extremely powerful means for providing such identifications and, in addition, for establishing a proteome-wide profile of all proteins whose abundance levels or phosphorylation state are affected by the drug.