DNA was extracted from exactly 25 mg of samples using DNeasy® Tissue Kit (Qiagen, Hilden, Germany) before and after storage. A nanodrop spectrophotometer (Thermo Scientific-2000, US and Canada) was used to determine the concentration of DNA in the solution. The samples were exposed to ultraviolet light at 260 nm and 280 nm. 260/280 ratio was used to control the purity of nucleic acids, and the amount of DNA was calculated with the formula: DNA concentration = OD260 × extinction coefficient (50 μg/ml) × dilution factor. All of the DNA samples were adjusted to just 20 ng by elution buffer (AE) and were stored at +4 °C until the next treatment