CTSK and breast cancer: Osteoclast formation requires the transcription factor nuclear factor of activated T cells 1 (NFATC1) and is accompanied by the subsequent production of genes including cathepsin K (Ctsk) and Trap. 26 To determine how CM from LIV-treated breast cancer cells reduced osteoclast formation, RNA from RAW 264.7 cells was examined by qPCR following exposure to CM.