Protein domains are independently folded regions of proteins that are often involved in protein-protein interactions. They are good candidates for the generation of protein microarrays because of their small size, their globular structures, and the fact that they are the protein-interacting workhorses of the cell. In addition, arrayed recombinant protein domains retain much of their binding specificity. Such microarrays can be probed with other recombinant proteins or fluorescently labeled peptides to identify potential binding partners and also determine how posttranslational modifications influence specific interactions. Thus, protein domains provide us with a system-oriented array that is focused on elucidating signal transduction pathway specificity and regulation.