We therefore mutated F126 in FOXRRM to histidine, tyrosine and leucine. We have previously shown using SPR that aromatic amino acids at position 126 are crucial for binding FOXRBE (F126Y: Kd =2.21 nM; F126H: Kd =25.9 nM), although a sterically-fitting aliphatic amino acid such as leucine can partially substitute for the phenylalanine (F126L: Kd =374 nM) . We irradiated these variants in the presence of FOXRBE. All three mutants cross-linked to FOXRBE, as evident from SDS-PAGE (Fig. ).