INTRODUCTIONThe Nucleic Acid Programmable Protein Array (NAPPA) approach for producing protein microarrays uses cell-free extracts to transcribe and translate cDNAs encoding target proteins directly onto glass slides. Following array preparation, the identification of protein interactions using NAPPA can be accomplished by either of two general schemas. The first is to probe an expressed NAPPA slide with a purified protein of interest (the query protein) and look for interactors. Signals of these interactions can be detected either by directly labeling the query protein or by using a labeled antibody either to the protein itself or to a tag on the protein. This approach works well when there is access to the purified protein, and it has the advantage that users can test query protein binding with and without post-translational modifications or under a variety of binding conditions. The second schema entails coexpressing the query protein on the NAPPA slide at the same time that all the target proteins are expressed. This article describes the advantages of using a coexpressed query protein and provides advice on choosing a suitable epitope tag.