  We study mathematically a method for discovering which gene is related to a
cell phenotype of interest. The method is based on RNA interference -- a
molecular process for gene deactivation -- and on coupling the phenotype with
fluorescence in a FACS machine. A small number of candidate genes are thus
isolated, and then tested individually. We model probabilistically this
process, prove a limit theorem for its outcome, and derive operational
guidelines for maximizing the probability of successful gene discovery.
