1. Kegg Markup Language manual, http://www.genome.ad.jp/kegg/docs/xml
2. KEGG pathway database - Kyoto University Bioinformatics Centre, http://www.genome.jp/kegg/pathway.html
3. Petri Net Markup Language, http://www.pnml.org
4. Petri net tools, http://www.informatik.uni-hamburg.de/TGI/PetriNets/tools
5. Taxonomy - site guide - NCBI, http://www.ncbi.nlm.nih.gov/guide/taxonomy/