Animals and tissue sample collection
Three Chinese Holstein cows which were in the same period of lactation were selected from Beijing Sanyuan Dairy Farm Center. All of them were fed in a consistent environmental condition. Eight tissues samples (heart, liver, lung, kidney, mammary gland, ovary, uterus and muscle) from each cow were collected within 30 min after slaughter and stored at liquid nitrogen. The whole procedure for collection of the tissue samples of all animals was carried out in strict accordance with the protocol approved by the Animal Welfare Committee of China Agricultural University (Permit number: DK996).