The genes expression levels in different months of testicular tissues. A, relative expressions of miR-122 and miR-449a in 1-day, 12-months-old and 24-months-old bull testes tissues; B, the mRNAs expressions of PDK4, DUSP4 and FKBP1B in 1-day, 12-months-old and 24-months-old bull testes tissues (**p<0.01).