NEW YORK--(BUSINESS WIRE)--Fitch Ratings upgrades one class of GMAC Commercial Mortgage Securities, Inc. commercial mortgage pass-through certificates series 2006-C1. A full list of rating actions follows at the end of this ratings action commentary. KEY RATING DRIVERS The upgrade is a result of increased credit enhancement due to additional paydown from loan maturities and additional defeasance since Fitch's last rating action. Fitch modeled losses of 22.4% of the remaining pool; expected loss