Within the peaks corresponding to the VP proteins, we identified seven proteoforms in AAV6, nine proteoforms in AAV8, and six proteoforms in AAV9 (Table ). All proteoforms were identified in both the empty and full capsids of their respective serotypes with the exception of a monophosphorylated VP2 proteoform in AAV9 that was only detected in the full capsids. Almost all identifications were made within a 10 ppm mass error, and all were made within a mass error of 20 ppm. As expected, the (Ac)VP1, VP2 and (Ac)VP3 proteoforms were all detected in each AAV serotype analysed. It is well understood that during production, the VP1 and VP3 proteins undergo N-term methionine (Met, M) cleavage at M1 and M203 (M204 for AAV8) respectively, and are then subsequently acetylated (Ac) resulting in the (Ac)VP1 and (Ac)VP3 sequences starting at the M + 1 amino acid . For AAV6, AAV8, and AAV9 the M + 1 amino acid is alanine (Ala, A) generating VP1 and VP3 proteoforms starting at A2 and A204 (A205 for AAV8), respectively. These acetylated proteoforms were confirmed through the presence of acetylated peptides starting at A2 and A204 (M205 for AAV8), respectively, detected during peptide mapping (AAV6: Table and Figures and, AAV8: Table and Figures S8 and S10, AAV9: Table and Figures and). Meanwhile, VP2 for all the serotypes commences at A139 and does not contain any N-term acetylation. This was also confirmed by the presence of peptides starting at A139 with peptide mapping (AAV6: Table and Figure , AAV8: Table and Figure , AAV9: Table and Figure ).