There were also some low abundant unknown components detected that could not be considered HCPs or other cellular contaminants due to their presence in both the empty and full capsids of their respective serotypes. In AAV6 this was AAV6 Unknown Component ). The exact nature of these components could not be determined, but all are thought to be a type of modified VP3 proteoform. AAV6-UC3, AAV8-UC2, and AAV9-UC1 are considered to be caused by the loss of a carboxyl group (COOH) from (Ac)VP3, unacetylated VP3, and (Ac)VP3, respectively. Most likely a neutral mass loss during CE-MS analysis as there is no difference in migration time between the aforementioned proteoforms and the aforementioned unknown components in their respective serotypes. AAV8-UC2 was initially thought to be a phosphorylated version of the un-acetylated VP3, but as discussed later, the migration time shift associated with phosphorylation was not present to suggest this was the case.