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. Identification of protein interactions can be accomplished either by probing an expressed NAPPA slide with a purified protein of interest (the 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 detection of query protein following coexpression of the query protein on the NAPPA array.