Reverse vapor diffusion can sometimes be used to crystallize a protein in low ionic strength. This is an alternative method to dialysis that is less tedious to set up but also allows the protein concentration to be become more dilute as the experiment proceeds, whereas dialysis can maintain a constant protein concentration. Begin with the concentrated protein in salt. The concentration of salt should be sufficient to maintain the solubility of the sample, perhaps 0.1 to 0.5 M salt. Set the sample for crystallization using the hanging or sitting drop vapor diffusion method. Pipet deionized water into the reagent reservoir. Pipet the high ionic strength protein sample onto the cover slide (hanging drop) or post (sitting drop). Do not add water to the sample. Using only deionized water in the reagent reservoir will allow water to diffuse from the reagent reservoir into the drop. As the drop size increases by vapor diffusion the ionic strength in the drop will decrease. The protein concentration will also decrease, so it is a good idea to begin with a concentrated protein sample. It is also helpful to avoid having too much salt in the samples since this may cause the sample drop to absorb too much water and dilute the protein concentration too much for crystallization. For cryo protection one can add glycerol to the reagent reservoir and mix this with the sample drop.