Make Sure That RNase A Is Added Into MX1 Buffer. Store The MX1 Buffer At 4˚C. If RNase A-Added MX1 Buffer Is Not Properly Stored At 4˚C Or Has Been Stored For A Long Time, E.G., More Than 6 Months, RNase A Activity May Have Been Reduced, Thus Not Being Able To Degrade RNA Completely. In This Case, Fresh RNase A Has To Added Into MX1 Buffer With Final Concentration Of 50 Mg/Ml. Again, Store The Buffer At 4˚C.