Interrogating the genomes of tumor cells with genomic and proteomic methods is becoming a mainstay of modern cancer classification efforts. This notion is brought to a new level by a paper in the July 23 issue of Cell, in which the dynamic responses of leukemia cells to perturbation are cataloged by flow cytometry, and the leukemias classified in terms of their functional responses. This study paves the way for more systematic attempts to bring functional genomics to the study of human cancer.