We synthesized the three labelled mutants of *NGCAUGU (N=A, G, C; Table ), as well as the corresponding per-labelled control sequences *N*G*C*A*U*G*U. We first confirmed that the NGCAUGU variants bound to FOXRRM using SPR. In this assay, parent UGCAUGU bound strongly to FOXRRM with a Kd=4.1 nM. Substitution of the 5'-uridine reduced the strength of the interaction by 4-6 fold for the three variants (AGCAUGU: Kd =24.9 nM; CGCAUGU: Kd =22.5 nM; GGCAUGU: Kd =21.3 nM) (Fig. ). This was consistent with the NMR structure showing that the 5'-uridine of FOXRBE contributes to binding by π-stacking to F126 (Fig. ) .