The Protein Ontology: a structured representation of protein forms and complexes. - Darren A. Natale; Cecilia N. Arighi; Winona C. Barker; Judith A. Blake; Carol J. Bult; Michael Caudy; Harold J. Drabkin; Peter D'Eustachio; Alexei V. Evsikov; Hongzhan Huang; Jules Nchoutmboube; Natalia V. Roberts; et al. (Nucleic Acids Res., 2011)