To provide convenient usage, the database was developed in a user-friendly manner, while browse and search options were provided to access the information of prokaryotic phosphorylation sites in the database. Since the phosphorylation sites are identified in different residues and various species, two browse options including ‘Browse by residue types’ ( Figure 3 A) and ‘Browse by phyla’ ( Figure 3 B) were developed in the database. Here, the serine hydroxymethyltransferase in E. coli (strain K12) was selected as an example to describe the usage of browse and search options. In the ‘Browse by residue types’, the phosphorylated residues are shown in diagrams ( Figure 3 A). By clicking the diagram of tyrosine phosphorylation, the distribution of tyrosine phosphorylated phosphoproteins in various organisms is returned ( Figure 3 A). Then the tyrosine phosphorylated phosphoproteins in Proteobacteria could be listed in a tabular format with ‘UniProt Accession’, ‘Name/Alias’ by clicking the link of ‘ Proteobacteria’ ( Figure 3 C). In the option of ‘Browse by phyla’ ( Figure 3 B), the 11 phyla in two domains including bacteria and archaea are listed for users to browse the phosphoproteins ( Figure 3 C). Through clicking on the figure of ‘ Proteobacteria’ , the distribution of phosphoproteins for different modification residue types is shown ( Figure 3 C). Then the list of tyrosine phosphorylated phosphoproteins could be retrieved after clicking the link ‘Tyrosine’, while the detailed information for specific phosphoproteins is provided by clicking protein entry ( Figure 3 D).