In the United States, a pharmacy benefit manager (PBM) is most often a third party administrator (TPA) of prescription drug programs but sometimes can be a service inside of an integrated healthcare system (e.g.: Kaiser or VA). They are primarily responsible for processing and paying prescription drug claims. They also are responsible for developing and maintaining the formulary, contracting with pharmacies, and negotiating discounts and rebates with drug manufacturers. Fortune 500 employers and public purchasers (Part D, the Federal Employees Health Benefits Program) provide prescription drug benefits to the vast majority of American workers and retirees. There are fewer than 100 major companies in this category in the US.