Fig. 1

There were 2214 EFO-native classes in January 2010, and 3992 EFO-native classes in January 2015. Although EFO has significantly grown in its number of native classes, the number of imported classes has grown at a much higher rate. Importing more than 6000 rare disease classes from ORDO in 2012, and axiomatizing them into EFO has resulted in a sudden increase between 2012 and 2013. This reflects the use of EFO as an application ontology providing interoperability across domain ontologies through semantic axiomatization