Fig. 2. Factors regulating osteoclast differentiation and function presented at this meeting. Stromal cells express RANKL, which activates RANK expressed in osteoclast progenitors. Activation of RANK induces expression of Fos and Fos-1, both of which are required for osteoclast differentiation. Estradiol-(E2) acts both on osteoblast and osteoclast precursors through ERα and ERβ. Osteoblasts also express the orphan receptor ERR, which could be regulated by E2. Osteoclasts express TCIRG1 and ClC-7, which are required for acidification and therefore resorption by osteoclasts.