“…This enzyme is also able to polymerize transcripts containing 4 -thiopyrimidines ( Figure 1.2a), a modification that increases their stability by about 50-fold relative to unmodified RNA (Kato et al, 2005). It was recently reported that the combination of mutated T7 RNA polymerases, Y639F and Y639F/H784A, allows the efficient incorporation of all four 2 -O-methyl nucleotides (Chelliserrykattil and Ellington, 2004;Burmeister et al, 2005Burmeister et al, , 2006. The positions that remain unmodified at the end of the in vitro selection procedure (e.g., the purine residues in a selection carried out with 2 -fluoropyrimidine triphosphates) can be modified post-SELEX for further optimization of the aptamers.…”