Resolving the structural basis of kinesin gating constitutes an additional challenge. In the microtubule-bound kinesin dimer, the neck linker in the front and rear head points backward and forward, respectively, and these different positions might regulate nucleotide- (Guydosh and Block, 2006, Tomishige et al., 2006 and Uemura and Ishiwata, 2003) or microtubule-binding affinity (Kawaguchi and Ishiwata, 2001). Intramolecular strain might also be transmitted through the neck linkers when they are stretched to allow the two kinesin heads to bind simultaneously to the microtubule (Figure S1 available online). Evidence that the neck linker position or strain mediates kinesin gating is primarily based on differences in the ATPase properties of wild-type kinesin dimer and truncated kinesin monomers (Hancock and Howard, 1999 and Rosenfeld et al., 2003), and certain point mutants (Klumpp et al., 2004); however, these reported changes in ATPase activity might be due to differences in the constructs rather than neck linker mediated strain.