a, EM map densities of the mRNA in the termination complexes containing the UAA, UAG and UGA stop codons reveal that they adopt the same compacted conformation. The ValGUU codon in the P site and the stop codon (+1 to +3) and following (+4) bases in the A site (purple) are indicated. b, The core termination signal recognised by eRF1AAQ (purple) is formed by four mRNA bases (+1 to +4, slate) that occupy the A site. Bases +2 and +3 stack on A1825, which is flipped out of helix 44 (h44), and base +4 on G626 of 18S rRNA (yellow). c, In bacteria, RF1 (grey) recognises a more extended stop codon configuration where the +3 base stacks on G530 (the equivalent of G626) of 16S rRNA.