In order to determine whether these findings apply more broadly to RNA-protein cross-linking, we analysed data from four published structures in which the required resolution of nucleotide/amino acid cross-linking was available. For example, Panhale et al. localised a cross-link in hnRNPC around a uridine π-stacked with F19 (Fig. ). In a novel approach, Lelyveld et al. used O-RNA labelling and targeted mass spectrometry to localise the cross-link of U11 in a let-7 microRNA precursor to a πstacked phenylalanine (F55) in the Lin28 cold shock domain (Fig. ) . Kramer et al. used a combination of cross-linking and mass spectrometry to pin-point cross-linking sites on amino acids from yeast ribosomal protein S1 . The data was correlated with published crystal structures of the protein, which showed that tryptophan (W117) participates in a π-stacking interaction with uridine U1799 (PDB 4V88) (Fig. ) . Finally, our own CLIR-MS data from PTBP1 in complex with internal ribosomal entry site (IRES) of encephalomyocarditis virus (EMCV) provided additional evidence for the role of aromatic amino acids in cross-linking reactions. Cross-links occur mainly at four aromatic amino acids (Y127, Y267, H411, H457), and mostly with uridines. Cross-comparison with the NMR solution structure of PTBP1 in complex with short polypyrimidine sequences shows that these amino acids are π-stacked to nucleotides (PDB IDs: 2AD9, 2ADB, 2ADC) (Fig. ) . In most cases, there is cross-linking not only to the stacked amino acids but also to flanking amino acids, such as Q412.