Additionally, our double-labeling immunostaining studies revealed the colocalization of the MT1 and MT2 melatonin receptors with the vascular endothelial marker CD31, the chondrogenic differentiation marker SOX9 and the osteogenic differentiation marker OCN in injured Achilles tendons during HO formation, suggesting that melatonin receptors, especially the MT2 melatonin receptor, mediate the effects of melatonin in modulating HO formation in rats with Achilles tendon injuries. Here, SOX9 is linked to Achilles tendon injury.