The invention relates to methods and systems (e.g., computer systems and computer program products) for characterizing cellular constituents, particularly genes and gene products. In particular, the invention provides methods for assigning or determining the biological function of uncharacterized genes and gene products by using “response profiles,” i.e., measurements of pluralities of cellular constituents in cells having a modified gene or gene product, as phenotypic markers for the gene or gene product. Methods are provided for clustering such response profiles so that similar or correlated response profiles are organized into the same cluster. The invention also provides databases or “compendiums” of response profiles to which the response profile of an uncharacterized gene or gene product can compared.