The Study Drug - Rebif® New Formulation (RNF) Rebif® is a medicine that is part of a family of proteins called interferon beta-1a (IFN-β-1a) molecules that play an important role in the immune system and help limit the damage that occurs with multiple sclerosis (MS). The interferon in Rebif® is like your body's own natural human interferon, but is made outside the body by a process called "recombinant DNA technology". Merck Serono International S.A. (the maker of Rebif®) has recently updated the method to make Rebif®, and it is referred to as Rebif® New Formulation (RNF).