Proteomics is the large-scale study of proteomes. A proteome is a set of proteins produced in an organism, system, or biological context. We may refer to, for instance, the proteome of a species (for example, Homo sapiens) or an organ (for example, the liver). The proteome is not constant; it differs from cell to cell and changes over time. To some degree, the proteome reflects the underlying transcriptome. However, protein activity (often assessed by the reaction rate of the processes in which the protein is involved) is also modulated by many factors in addition to the expression level of the relevant gene. The EBI hosts up-to-date and accurate databases to enable rapid searching and retrieval of these data. The four major databases related to proteomic research (UniProtKB, IntAct, Reactome and PRIDE) are described in the next section. These four databases (especially UniProtKB) draw from gene sequence data (e.g. Ensembl) and annotation tools (e.g. InterPro) also hosted by the EBI. You can find out more about such resources in other training courses, such as the Introduction to Functional Genomics Resources, and the InterPro Quick Tour.