US infringement defense under section 273 of the Patent Act (35 U.S.C. § 273) which applies to infringement of method claims, especially in business method patents. The defense was created by Congress in 1999 in response to the State Street Bank case which allowed business method patents. The purpose of the defense is to allow businesses to show that they were using a patented method 1-year before the filing date of the patent and also to use by non-profit organizations. The defense is “personal” i.e., it cannot be licensed or assigned to another, it does invalidate the patent, and if a business benefiting from the defense is sold, use of the defense is limited to sites where the invention was used before the effective filing date of the patent or the date of the assignment, whichever is later.