Table 1.

Subnetworks of the Disease Ontology

Root disease	#Diseases (nodes)	#Edges	#Nodes with 1 parent	#Nodes with 2 parents	#Nodes with 3 parents
Disease	2,039	2,095	1,982	55	1
Cardiovascular disease	141	141	139	1	0
Gastrointestinal disease	115	118	110	4	0
Musculoskeletal disease	133	135	129	3	0
Nervous System disease	308	324	291	15	1

The entire Disease Ontology (root = “Disease”) and four subnetworks of various sizes extracted from it. The original DO and its subnetworks are tree-like: 1) the numbers of edges are close to n−1, where n is the number of nodes and 2) only a small fraction of nodes have 2 or more parents