F-to-E/Q/K replacement pathways. Start and end residues are black with white codons, intermediate residues are gray with black codons. Codons are written using DNA symbols (A, adenine, T, thymine, C, cytosine, G, guanine, Y, pyrimidine, R, purine). Red pathways cross two different residues; blue pathways cross one residue; dashed pathways cross a stop codon. Note that dashed paths are identical in (A) F-to-E, (B) F-to-Q, and (C) F-to-K.