RNA binding by the human PHAX-RBD. (A) 15N-labeled human PHAX-RBD (0.2 mM) was titrated with different RNA ligands: U9 (black), U4 (red), AUCG (yellow), A14 (blue), SmRNA–AAUUUGUGG (green). Saturation is observed at a 1:1 molar ratio. (B) 1H,15N pairwise chemical shift differences Δδ = {[Δδ(1H)]2×5) + [Δδ(15N)]2}1/2 of the data shown in A versus primary sequence.