A common PTM of AAVs is N-terminal acetylation. It is known that in nature over 80% of human proteins are co-translationally N-term acetylated . Proteins with N-term Met are often cleaved by Met-aminopeptidases when the amino acid at position 2 is alanine, valine (V, Val), serine, threonine (T, Thr), cystine (C, Cys), glycine (G, Gly), or proline (P, Pro), due to their small side chains .