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, interactions with a protein of interest (query protein) are detected either by probing an expressed NAPPA slide with the purified query protein or by coexpressing the query protein on the NAPPA slide at the same time that the target proteins are expressed. This protocol describes the coexpression method, which involves adding the gene for the query protein to the cell-free protein expression mix. The amount of query protein that is transcribed and translated from the corresponding plasmid DNA depends on the amount of plasmid DNA used and the size of the protein of interest, among other factors. If too little query protein is expressed, there may be no detectable binding signal. Excessive amounts of protein expression may generate nonspecific background signals. Because the optimum amount of query plasmid varies with each query protein, it is essential to assess empirically the optimal amount of query protein DNA to add to a coexpression experiment.