--- - "\nAbstract\nExcessive osteoclast formation and function are considered as the main causes of bone lytic disorders such as osteoporosis and osteolysis. Therefore, the osteoclast is a potential therapeutic target for the treatment of osteoporosis or other osteoclast‐related diseases. Helvolic acid (HA), a mycotoxin originally isolated from Aspergillus fumigatus , has been discovered as an effective broad‐spectrum antibacterial agent and has a wide range of pharmacological properties. Herein, for the first time, HA was demonstrated to be capable of significantly inhibiting receptor activator of nuclear factor‐κB ligand (RANKL)‐induced osteoclastogenesis and bone resorption in vitro by suppressing nuclear factor of activated T cells 1 (NFATc1) activation. This inhibition was followed by the dramatically decreased expression of NFATc1‐targeted genes including \nCtr (encoding calcitonin receptor), \nAcp5 (encoding tartrate‐resistant acid phosphatase [TRAcP]), \nCtsk (encoding cathepsin K), \nAtp6v0d2 (encoding the vacuolar H+ ATPase V0 subunit d2 [V‐ATPase‐d2]) and \nMmp9 (encoding matrix metallopeptidase 9) which are osteoclastic‐specific genes required for osteoclast formation and function. Mechanistically, HA was shown to greatly attenuate multiple upstream pathways including extracellular signal‐regulated kinase (ERK) phosphorylation, c‐Fos signaling, and intracellular Ca\n2+ oscillation, but had little effect on nuclear factor‐κB (NF‐κB) activation. In addition, HA also diminished the RANKL‐induced generation of intracellular reactive oxygen species. Taken together, our study indicated HA effectively suppressed RANKL‐induced osteoclast formation and function. Thus, we propose that HA can be potentially used in the development of a novel drug for osteoclast‐related bone diseases." - Journal of Cellular Physiology, EarlyView.