Isoelectric focusing is the electrophoretic method that separates proteins according to their iso-electric points. The haemoglobin molecules move to the point in a pH gradient where the net charge on the molecule is zero. This gives much sharper bands and better resolution for the separation of different haemoglobins than other electrophoresis methods, allowing very accurate measurement of band positions to within 1mm apart. Because many variants move to a characteristic gel position, the isoelectric method is used to make a presumptive diagnosis of some of the more rare Hb variants. Note that all electrophoresis methods (and HPLC) only give a presumptive diagnosis; a definitive identification requires characterization by DNA sequencing or mass spectrometry (except for the sickling Hbs).