My interpretation, though it doesn't go into it in the article, is that ribosomes are capable of translating abnormal tRNA bases to abnormal amino acids; they j

My interpretation, though it doesn't go into it in the article, is that ribosomes are capable of translating abnormal tRNA bases to abnormal amino acids; they just normally don't do it because there aren't tRNA strands floating around with abnormal bases on them. They happened to find an abnormal DNA/RNA base pair, and it happened to be mapped by the ribosome to a PEG amino acid (I think the implication is NH2-PEG-COOH?), and relized if they swapped out an existing amino acid in IL-2 with that PEG-based amino acid, the protein would be less toxic? Another thought: it might not be strictly a RNA-base-X maps to amino-acid-x type of operation; RNA-base-X might map to either amino-acid-x or amino-acid-y based on concentrations of AA-x and AA-y, or even based on neighborhood structure of the protein that's under construction, and they just got lucky with IL-2 or they figured they could put tons of PEG into a cell and get most IL-2 produced with that PEG-based amino acid?