CytoSelect WST-1 Cell Proliferation Assay Reagent is a clear, slightly red, ready-to-use solution and should be stored at 4 degree C. If precipitates or turbidity are observed upon thawing, warm the solution to 37 degree C for 5-10 minutes and agitate to dissolve the precipitates. For longer periods, aliquot and store at -20 degree C protected from light to avoid repeated freeze-thaw cycles.