the subject and object and is also represented by a URI. For example, in the KEGG database the breast cancer gene hsa:675 encodes the Homo sapiens protein with the number 119395734 in NCBI Protein database. This gene is the same as gene ENSG00000139618 in Ensemble database. This could be expressed as two triples: " . ". The relationship among these three resources: hsa:675, protein:119395734, and ENSG00000139618, in three databases is established. Similar to how any document expressed in HyperText Markup Language (HTML) can be linked to any other document expressed in HTML, the information expressed in RDF can be connected to any other information expressed in RDF (26). However, with respect to HTML, a linked resource must be a whole document, whereas with RDF, any information defined as a resource can be linked together. RDF is expressive with the simple triple format. The Semantic Web integrates not only resources that are themselves built or represented using RDF but also those resources that can be mapped to RDF (29).