Triosephosphate isomerase (TIM, EC 5.3.1.1) catalyzes the interconversion of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, the fifth reaction of the glycolytic pathway. Its active site loop exhibits a hinged-lid motion, alternating between the two well-defined "open" and "closed" conformations. In a previous work, three crystallographic structures of apo rabbit muscle TIM (a dimer of identical subunits, 2 x 26.6 kDa) were determined either in the presence or absence of DMSO. In the former situation, the active site loop in one of the subunits was in the closed conformation, a surprising finding as this conformation is usually observed only in complexes with substrate analogues (Aparicio et al., 2003, J. Mol. Biol. 334:1023-41). Intriguingly, removal of DMSO from the crystallization conditions resulted in a crystal in which the active site loop was in the open conformation in all the subunits of the crystal ASU. No DMSO molecules were observed near the lid of the closed loop or at the active site. However, DMSO is able to modify solvent properties and could be related to the loop movement and its stabilization in the closed conformation. In this work, we present a new structure of apo rabbit TIM, determined in the same crystal form as before, from crystals grown in the presence of DMSO. Preliminary results indicated that, in this new structure, the active site loop is in the open conformation in all the subunits, suggesting that the loop closure is not connected to the presence of DMSO. Further refinement and structural analysis are currently in progress.