The cover art depicts the RNA catalyst ‘flexizyme’ based on its three-dimensional structure. It attaches a yellow sphere representing a non-canonical amino acid onto a tRNA, which is shown as a classical two-dimensional ‘cloverleaf’ representation. The catalyst is aided by two structural magnesium ions represented by small, darker red circles. Surrounding the flexizyme is a pool of acylated tRNA molecules and numerous macrocyclic peptides incorporating the non-canonical amino acids shown as colored circles.