Inspection of the m/z region corresponding to a isotypes of ChET revealed that, in addition to the full-length peptide (a1CT: m/z = 2861.80) and the detyrosinated peptide (a1CT-Y: m/z = 2697.85), there was an unidentified peak in between these signals (a1CT-Y+'X' : m/z = 2805.09). This observation is expanded in detail in Figure . The mass of this analyte does not match that of any CNBr peptide predicted from the sequence or known PTMs. Essentially identical results were obtained from multiple preparations of ChET, prepared at NIH and at Binghamton University.