The function of an uncharacterized gene can sometimes be determined by mutating the gene and using a phenotypic assay But sometimes a convenient phenotype does not exist for a given cellular function. Hughes et al. suggest in the July 7 Cell that expression profiles can be used instead (Cell 2000, 102:109-126). Rather than measuring expression profiles as conditions change (e.g., at different points in the cell cycle, Hughes et al. keep the culture conditions constant and measure the profiles of 276 deletion mutants, 11 conditional lethal mutants, and wildtype cells after each of 13 different chemical treatments. The resultant expression profiles are clustered and used to assign functions to 8 previously uncharacterized genes and to determine the target of a commonly used anesthetic. Many important correlated expression changes are small (1.5-fold or less), suggesting that data sets need to be highly internally consistent, and large.