sentence: For example , MK608 showed promising antiviral activity in chimpanzees ( 11 ) , but further clinical studies have not been reported to date.The mechanisms for toxicity of ribonucleotide analogs have not been well characterized but could theoretically arise from incorporation into cellular RNA by human RNA polymerases I , II , III ( PolI , PolII , and PolIII ) and the mitochondrial RNA polymerase ( PolRMT ) .