Figure 1. Schematic signaling for macrophage cell–cell fusion. GM-CSF activates STAT1 through the GM-CSF receptor (GM-CSFR), while IL-4 activates STAT6 through IL-4 receptor (IL-4R). Activation of STAT6 followed by STAT1 inhibition is required for the expression of DC-STAMP and OC-STAMP, both of which are essential molecules for macrophage and osteoclast cell–cell fusion. STAT6 is not involved in osteoclast cell–cell fusion.