Comparison between modern (reverse) and classical (forward) biochemical and genetic approaches. (a) Present-day techniques that enable the generation of strains each containing a different affinity-tagged gene means that all protein complexes containing the tagged protein can be subsequently identified. (b) A protein with an activity of interest can be purified from a crude protein extract (the total proteome) by rounds of chromatographic separation followed by assaying fractions for the biochemical activity. (c) An exhaustive collection of strains each with a different gene deleted can be tested in a single experiment to identify, for example, all genes essential for growth in a particular set of conditions. (d) Mutagenesis followed by breeding of a large population and subsequent screening for some predetermined phenotype will identify only a relatively small number of mutants in an individual screen.