A non-canonical amino acid (ncAA) is recognized by an orthogonal aminoacyl-tRNA synthetase and loaded onto its cognate tRNAXXX, where XXX decodes a sense codon. The orthogonal tRNA decodes its cognate sense codons in competition with the natural tRNAs, leading to ncAA incorporation at sense codons and a substoichiometric, co-translational incorporation of the ncAA in the proteome. The amino acid can then be fluorescently labeled in SORT-M (not shown) or used to enrich the tagged proteome, in SORT with enrichment of tagged proteins (SORT-E), as shown. In SORT-E, the cleavable, biotinylated probe 2 is covalently coupled to amino acid 1 that has been incorporated via SORT. Tagged proteins can then be captured by solid-upported streptavidin. After stringent washing, the linker is cleaved under mild conditions to elute the enriched target proteins. Figure adapted from a prior publication: Elliott, T.S., Bianco, A., Townsley, F.M., Fried, S.D. & Chin, J.W. Tagging and Enriching Proteins Enables Cell-Specific Proteomics. Cell chemical biology 23, 805–815 (2016).