In the United States, a pharmacy benefit manager (PBM) is a third-party administrator of prescription drug programs for commercial health plans, self-insured employer plans, Medicare Part D plans, the Federal Employees Health Benefits Program, and state government employee plans. According to the American Pharmacists Association, "PBMs are primarily responsible for developing and maintaining the formulary, contracting with pharmacies, negotiating discounts and rebates with drug manufacturers, and processing and paying prescription drug claims." PBMs operate inside of integrated healthcare systems (e.g., Kaiser Permanente or Veterans Health Administration), as part of retail pharmacies (e.g., CVS Pharmacy or Rite-Aid), and as part of insurance companies (e.g., UnitedHealth Group).As of 2016, PBMs managed pharmacy benefits for 266 million Americans. In 2017, the largest PBMs had higher revenue than the largest pharmaceutical manufacturers, indicating their increasingly large role in healthcare in the United States. However, in 2016 there were fewer than 30 major PBM companies in this category in the US, and three major PBMs (Express Scripts, CVS Health, and OptumRx of UnitedHealth Group) comprise 78% of the market and cover 180 million enrollees.