Fluorescence anisotropy titrations previously established that N-Myc61-89 binds to Aurora-A with low µM affinity. Using either Trp/Ala or Glu/Lys variants, three residues in N-Myc61-89 were found to have a strong effect on binding affinity (PDB: 5G1X; Fig. ): tryptophan residues Trp77 N-Myc , Trp88 N-Myc and Glu73 N-Myc , which presumably participates in a salt bridge with Aurora-A via Lys143 AurA . The N-Myc/Aurora-A co-crystal structure suggests that residues 61-74 of N-Myc do not engage in specific non-covalent interactions with Aurora-A, except for Leu61 N-Myc , which seems to insert into the Aurora-A surface W pocket (not shown). We started these studies by probing the effects of truncating residues 61-74 on binding (Table ). A short N-Myc variant spanning the helical region (N-Myc73-89) was prepared and its IC50 value determined through competition against a fluorescein labelled N-Myc61-89 sequence (Table ).