T7 Flex Polymerase is a single mutant (Y639F) of T7 RNA Polymerase which allows the enzyme to incorporate both rNTPs and dNTPS, as well as 2´-Fluoro-dNTPs into full length transcripts much more efficiently than wild-type T7 polymerase while retaining promoter specificity and overall activity. Such modified transcripts are useful for many applications including synthesis of RNase resistant RNA.