The U.S. Department of Justice has initiated an antitrust investigation into UnitedHealth Group, the nation's largest health insurer, which also operates the health services arm, Optum. The investigation, reported by multiple sources including the Wall Street Journal, focuses on UnitedHealth's $372 billion revenue health insurance/benefit program and its Pharmacy Benefit Manager (PBM) operations. The probe has recently seen investigators interviewing healthcare industry officials about UnitedHealth's insurance unit and its relationships with Optum, as well as the potential impacts of the company's doctor-group acquisitions on rivals and consumers. Following the news, UnitedHealth's stock experienced a significant drop, falling more than 5% to $485.98. UnitedHealth has received a CID in the Managed Care Probe as DOJ seeks expert witnesses.