Deamidation of asparagine to aspartic acid or isoaspartic acid (Iso-D, Iso-Asp) is often considered to impact the function of biotherapeutics including their potency, efficacy and safety and thus is monitored as a CQA . While digestion was performed at low pH, which is shown to reduce deamidation formation , deamidation at N56 (N57 when including Met) was found to be present 63.25% of the time, over 13 times more abundant than the next incidence of deamidation. Given that the presence of deamidation at N56 in AAV2 has been shown to reduce transduction efficiency it is important to understand the high level of deamidation at this site. Previous studies have shown that the structure of amino acid residue following Asn (N+1) will influence the rate of deamidation, as its size and charge impact the local flexibility of the peptide backbone . Gly as the N+1 residue has been shown to increase the presence of deamidation the most with at least one AAV study showing >75% deamidation occurring at Asn residues when Gly is the N+1 residue . When examining the N+1 residues for all deamidated Asn detected during AAV2 peptide mapping, Gly was only present as the N+1 residue for N56, which would help explain its high relative abundance compared to other deamidation features present. Additionally, a recent study has shown that sample preparation conditions including denaturation temperature, digestion temperature and digestion duration can generate deamidation artifacts during peptide mapping that exaggerate the levels of deamidation present . Hence, it is possible that the sample preparation conditions for peptide mapping are generating deamidation artifacts that are contributing to the high levels of deamidation seen at N56 and inflating the levels of deamidation seen at lower abundances. While beyond the scope of this paper, these findings suggest more work is needed to determine how much of the detected Oxidation is a PTM monitored as a CQA during production, purification or long-term storage since its presence can adversely impact biotherapeutic stability, efficacy, and product safety . A prominent oxidation observed in biotherapeutics is at the sulphur on the methionine side chain to create methionine sulfoxide . Here, the only methionine oxidation with a relative abundance greater than 10% was found at M633 (10.24%). Additionally, four Met oxidations were detected at M401