Identification of translation initiation sites. (A) A schematic diagram for enrichment of protein N-terminal peptides using the TAILS method. All primary amine groups (α- and ε-) on proteins were blocked by dimethylation followed by trypsin digestion. Non-N-terminal peptides displaying a free, trypsin-generated α-amine group were depleted using HPG-ALD polymer, and the remaining N-terminal peptides were analyzed in mass spectrometry after bRPLC fractionation. (B) The acetylated peptides mapping to annotated translation initiation sites were identified from HEK293T cells, HUVEC, human colon, and human substantia nigra using the TAILS method. (C) Overlap of the acetylated peptides mapping to annotated translation initiation sites from HEK293T cells, HUVEC, human colon, and human substantia nigra.