Monophosphorylated proteoforms of each VP ((Ac)VP1 + 1´P, VP2 + 1´P and (Ac)VP3 + 1´P) were identified in all serotypes examined, with the exception of (Ac)VP1 of AAV9, where only the unphosphorylated proteoform was seen. Additionally, a diphosphorylated VP1 ((Ac)VP1 + 2´P) proteoform was identified in the AAV8 serotype. The presence of phosphorylation in each serotype was confirmed via peptide mapping. As expected, in both the AAV6 and AAV9 serotypes, the phosphorylated proteoforms for each VP were in low abundance compared to their unphosphorylated counterparts. However, with AAV8, the predominant proteoform for both VP2 and (Ac)VP1 was the phosphorylated proteoform. Peptide mapping of AAV8 revealed high levels of phosphorylation between valine V132 and aspartic acid D185 (V132-D185) of VP1 (Table ). As VP2 starts at A139, any phosphorylation detected after A139 would be present on both (Ac)VP1 and VP2. A significant amount of phosphorylation was found to be present near threonine T138 in the empty capsids (46.44%), a region unique to VP1. However, the predominant source of phosphorylation found on AAV8 is around serine S153 for the empty capsids (62.70%) and S149 for the full capsids (87.53%), though BPF 5.1 could not determine the exact residue of the phosphorylation in either case. The inability of BPF to determine the exact phosphorylation location might be because phosphorylation has been shown to impact enzymatic digestion efficiency , which would explain why the phosphorylated peptides detected within the V131-D184 region are 20 -53 amino acids in length.