The site-specific resolution provided by the CLIR-MS technique, and in several published structures, has highlighted the role of π-stacking interactions as a pre-requisite for cross-linking of the FOXRRM-FOXRBE interaction. Thus, U1, G2 and G6 yielded "direct" cross-links with the two phenylalanines with which they stack, and "indirect" cross-links with 1-3 amino acids flanking the phenylalanines. Most notably, nucleobases that do not stack to aromatic side chains, did not partake in cross-linking.