Superposition of apo- (yellow) and holo-CRABPII structures. The CRABPII·RA complex (protein in green and RA in red. Kleywegt, Bergfors, Senn, Le Motte, Gsell, Shudo, and Jones. Structure 1994; 2:1241-58) is superimposed with the apo-form (Chen, Tordova, Gilliland, Wang, Li, Yan, and Ji. J. Mol. Biol. 1998; 278:641-53. Abstract) by structure-based alignment. The view is along the long axis of RA into the binding cavity.