have no mobile protons while the peptide with relatively low abundance (<6%) has mobile protons (Table ). Therefore, it is possible that some, if not most, of the detected glutamine NH3 loss, especially at Q100 and Q119, is produced during sample preparation and MS analysis instead of being a process induced PTM. However, further studies are necessary to determine what portion of the detected NH3 loss can be attributed to each possibility and are beyond the scope of this paper. Other PTMs of note identified above 1% relative abundance were the presence of succinimide on asparagine residues D326 (4.26%), D344 (2.77%), and D624 (5.79%), as well as phosphorylation at serine residue S148 (4.43%). Succinimide is the ringed intermediate formed during the deamidation of Asn to Asp through the loss of NH3 (from now on referred to as succinimide N) or through the loss of water from Asp residues (succinimide D) and can negatively influence the efficacy of biotherapeutics . While unstable at physiological conditions, succinimide is stable at low pH and thus its detection is not surprising. Phosphorylation is known to play a critical role in a variety of cellular processes with preliminary research suggesting it might play a role in AAV transduction efficiency .