Optimal use of informatics tools (e.g., tools for analyzing data, etc.) and resources (e.g., databases, data sets, etc.) depend upon explicit understandings of concepts related to the data upon which they compute. This is typically accomplished by a tool or resource adopting a formal controlled vocabulary and ontology. For the purpose of this Funding Opportunity Announcement (FOA), an ontology is defined as a controlled vocabulary that describes objects and the relationships between those objects in a formal way. Generally, an ontology has a grammar that allows the terms of the vocabulary to express something meaningful to the biomedical researcher. In an effort to advance the use of powerful informatics approaches in biomedical research, this FOA solicits Research Project Grant (R01) applications from institutions/ organizations that propose to develop an ontology that will make it possible for software to understand how two or more existing data sets relate to each other.