Analysis of the aforementioned CLIR-MS data (Fig. , Fig. ) had provided two important insights: i) on the RNA side, strong cross-linking only occurred with nucleotides that were stacked to aromatic amino acids (F126, F160, Fig. ); and ii) on the protein side, whereas the cross-links involved F126 and F160, cross-links were also detected at one to three amino-acids up-and downstream of F126 and F160 (positions 124-127, 158-164).