Total RNA from the tissues samples was extracted using an Axygen total RNA extraction kit (Axygen, USA) as described in the manufacturer's instructions. Briefly, 400 ml of lysis buffer was added to 30-40 mg of ground tissue. The nucleic acids were eluted in 50 ml nuclease-free water and stored at 220uC. For every five samples, one negative control (water) was included to detect any possible contamination.