Analysis of in silico digests of a theoretical proteome composed of UniProt and NTR sequences.A, in shotgun proteomics, UniProt peptides dominate the digest, leaving marginal chances to identify unique NTR peptides (1.4–3.7%, dependent on the protease used). B, enrichment of N-terminal peptides enhances NTR identification. C, most UniProt proteins (>98%) produce at least one unique peptide, whereas >30% of NTR proteoforms cannot be uniquely identified. D, enrichment of N-terminal peptides may lead to more inconclusive identifications than using a shotgun proteomics approach. NTR, noncoding transcript.