Interestingly, one of these π-stacked interactions (H457) appear to be with cytosine. Although other groups have described that cytosine participates at a low level in cross-linking , we did not observe more than minor amounts in mass spectral analyses in this study . ) . Binding affinities of FOXRBE are taken from ref . c) Illustration of the localised cross-link with the solution structure i) for hnRNP C binding on AUUUUUC obtained by Cienikova et al. (PDB 2MXY). ii) for Lin28A in complex with preEM-let-7f obtained by Nam et al. (PDB 3TS0). iii) for 40S ribosomal protein S1 in complex with the 18S rRNA obtained by Ben-Shem et al. (PDB 4V88) visualized with PyMOL (The PyMOL Molecular Graphics System, Version 2.5 Schrödinger, LLC).