bis-ANS is a high-affinity non-covalent extrinsic fluorescent dye used to analyze protein conformation.[1] Its predominant interaction with proteins is through its hydrophobic phenyl and naphthyl rings.[2] bis-ANS has an excitation maxima of 390 nm.[3] It has an emission maximum of 523 nm when free in solution but undergoes a blue shift with an increase in fluorescence intensity when bound to protein; for example, when bound to intestinal fatty acid binding protein (FAPB2) it has emission maxima of 484-496 nm. bis-ANS has been used to label mechanically damaged neurons in acute brain slices.4 It also potently inhibits microtubule assembly.[5],[6]