A method for identifying a cell or strain of cells containing a mutation in a gene involved in growth, comprising the steps of forming a labeled set of strains comprising a plurality of members, each member of the set containing an exogenous DNA fragment of a defined length stably integrated into the chromosome of a member, the defined length in each member differing from the defined length in other members, subjecting the labeled set of strains to mutagenesis so as to obtain mutants from each member of the set of strains, and introducing the mutant strains into a growth environment for a period of time sufficient for growth of a non-mutated strain and determining which strains have reduced growth compared to a non-mutated strain, by determining the presence and size of exogenous DNA fragments relative to each other using PCR and agarose/polyacrylamide gel electrophoresis.