Further evidence of the estrogen-bone link is based on the presence of estrogen receptors (ER) on bone cells and targeted deletion studies. ERα is found on osteoclastic and osteoblastic cells, while ERβ is found on osteoblasts . Targeted deletion of osteoblastic ERα in murine models led to low bone mass in both males and females . The targeted deletion of ERα in osteoclasts and osteoclast progenitor cells increased osteoclast numbers in females but not in males . Another study of ERβ deletion in mesenchymal stem cells found that bone mass increased only in female rodents . These knockout studies indicate that estrogen signaling is vital to bone homeostasis in males and females, with sex-based differences in these signaling mechanisms.