A fairly large set of protein interactions is mediated by families of peptide binding domains, such as Src homology 2 (SH2), SH3, PDZ, major histocompatibility complex, etc. To identify their ligands by experimental screening is not only labor-intensive but almost futile in screening low abundance species due to the suppression by high abundance species. An ideal way of studying protein-protein interactions is to use high throughput computational approaches to screen protein sequence databases to direct the validating experiments toward the most promising peptides. Predictors with only good cross-validation were not good enough to screen protein databases. In the current study we built integrated machine learning systems using three novel coding methods and screened the Swiss-Prot and GenBank TM protein databases for potential ligands of 10 SH3 and three PDZ domains. A large fraction of predictions has already been experimentally confirmed by other independent research groups, indicating a satisfying generalization capability for future applications in identifying protein interactions. Experimental screening for protein binding peptides is not only labor-intensive but almost futile in screening for low abundance binding species due to the suppression by high abundance species. A more plausible way of studying protein-protein interactions is by using high throughput computational predictions rather than experimental approaches to screen for interactions from protein sequence databases to direct the validating experiments toward the most promising peptides. A prediction can only be called successful when the overall efficiency and cost of computational prediction plus biological validation are much better than those of experimental screening. A fairly large set of protein interactions are mediated by families of peptide binding domains, such as SH2, 1 SH3, PDZ, MHC, etc. that act as receptors to accommodate, in their binding pockets, short peptides in an extended conformation (1, 2). We studied two common domainligand interactions mediated by SH3 domain and PDZ domain. SH3 domains selectively bind peptides of 8 -11 amino acids on their ligand proteins; PDZ domains bind 4 -8 amino acids in the C termini of their partners. Ligands of both PDZ and SH3 domains are of high diversity. Two good reviews provide detailed information on SH3 domain (3) and PDZ domain (4). Two major categories of methods have been developed to predict domain-ligand interactions. The first category is based on structure information as exemplified by the work on predicting MHC-specific epitopes with protein docking methods (5, 6). This type of method needs intensive computation and the prior knowledge of the three-dimensional structures of the bait proteins. Instead of using exact protein structures, Brannetti et al. (7) and Altuvia and Margalit (8) extracted ligandcontacting residues from the known domain-ligand complex structures to approximately represent domain interface structures of SH3 and MHC, respectively. The interactions of amino aci...