Representation of the electron density for the thallium ions in the suggested cation-binding site (Tl^Ct) and Na2 site (Tl2) of Glt_Ph (PDB code 4P1A ). The 2Fo-Fc map is colored in blue and contoured at 3σ. The Fo-Fc map is colored in green and red (±3σ). Difference maps are used to check the fit of the model to the diffraction data (see Glossary). The Fo-Fc difference map is a tool to visualize possible misfits and errors: positive peaks (green) indicate missing parts of the model, and negative peaks (red) indicate that these parts of the model are not supported by experimental data, and hence have to be removed. Additionally, negative density peaks might indicate inappropriate refinement of occupancies/B-factors and/or severe radiation damage. Cartoon representation; thallium ions are shown as brown spheres.