AR and Infertility: Although our previous work showed that the irregular expression of the AR and SRD5A2 in testicular Leydig cells may be associated with cattle–yak infertility [14], we performed this study to evaluate the expression and significance of the AR and 3β-HSD in the epithelial cells of the hybrid cattle–yak epididymis and to determine their correlation with infertility in comparison with yak samples, because the sperm maturation processes in the epididymis are highly related to androgen responses.