Translation initiation sites mapping to 5′ UTRs identified by a unique database search strategy. (A) Substitution of near-cognate initiation codons to AUG enables identification of acetylated peptides that begin with methionine by searching against a customized database as shown. (B) Peptides acetylated at their N termini mapping to 5′ UTRs that were identified from HEK293T cells, HUVEC, human colon, and human substantia nigra through a database search against a customized database that incorporates substitution of near-cognate initiation codons to AUGs. The colors indicate whether the peptides correspond to upstream open reading frames (ORFs) or were N-terminal extensions of annotated proteins. (C) Codons for TISs corresponding to acetylated peptides mapping to 5′ UTRs identified through a search against a customized database that incorporates substitution of near-cognate initiation codons to AUG. (D) Sequence logo of nucleotides surrounding the TISs identified by searching against the database with the substitution of near-cognate initiation codons to AUG.