"The National Institutes of Health (NIH) is the primary federal agency charged with conducting and supporting biomedical and behavioral research. It is the largest of the eight health-related agencies that make up the Public Health Service (PHS) within the Department of Health and Human Services (HHS). NIH's organization consists of the Office of the Director (OD) and 27 Institutes and Centers. The OD sets overall policy for NIH and coordinates the programs and activities of all NIH components, particularly in areas of research that involve multiple institutes."
CRS Report for Congress, R43341
Congressional Research Service: https://crsreports.congress.gov/