RNA polymerase II (Pol II), a key enzyme in our gene expression, is responsible for transcribing DNA into messenger RNA. Errors in transcription can cause deleterious effect upon repeated translation of erroneous mRNA into protein. Transcription infidelity may result in aging and human diseases such as cancer. During transcription, Pol II can detect the mis-incorporated RNA and backtrack to correct errors to ensure that each messenger RNA created will match with template DNA. However, it remains largely a mystery how Pol II controls the fidelity of gene transcription.