Absence of electron density in the Na1 site for the apo Glt_Ph structure (PDB code 4OYF ). The electron-density map (2Fo-Fc) is shown as a blue mesh and contoured at 1σ. The Fo-Fc map is colored in green (3σ) and red (−3σ). See Glossary for explanation of 2Fo-Fc and Fo-Fc maps. Cartoon representation; sodium ion Na1 assigned in this structure is shown as a purple sphere, and amino acid residues supposedly involved in its coordination are shown as sticks.