The tandem affinity purification (TAP) procedure was pioneered in yeast for the purpose of purifying and characterizing protein complexes and has since been adapted for use in many organisms, including mammalian systems. The TAP procedure involves two sequential affinity purification steps to avoid non-specific protein interactions, a common problem in identifying proteins in complexes. Bimolecular Affinity Purification (BAP): Tandem Affinity Purification Using Two Protein Baits, from Ezra Burstein and colleagues, presents a variation on the TAP procedure in which the affinity moieties are placed on two different proteins of a molecular complex to isolate or detect components present in the complex. This variation, called bimolecular affinity purification (BAP), is suited for the identification of specific molecular complexes marked by the presence of two known components. The article is featured in the November issue of Cold Spring Harbor Protocols and like all our featured articles, is freely available to subscribers and non-subscribers alike.