Why only add two 2'-O-methyl nucleotides and phosphorothioates for the modification pattern to the 5′ and 3′ synthetic sgRNA ends (2xMS) instead of three modifications (3xMS) as depicted in Porteus et al?

We and other researchers have observed toxicity due to the presence of several phosphorothioate linkages (Deleavey and Damha, 2012). We compared the 2xMS and 3xMS modification patterns and observed similar functionality with reduced toxicity using a the 2xMS verses the 3xMS modifications.