From
http://www.usmint.gov/about_the_mint/mint_facilities/index.cfm?action=SF_facilities:
The United States Mint at San Francisco plays an important role in our nation's coinage. Although it does not currently produce circulating coins, it is the exclusive manufacturer of regular proof and silver proof coin sets that set the standard for numismatic excellence with their brilliant artistry, fine craftsmanship and enduring quality.