To investigate whether C/EBPβ-induced RANKL expression stimulated osteoclast formation, we co-cultured RA-FLS, which were transfected with adenovirus vectors expressing C/EBPβ-LAP, −LIP or LacZ control, and PBMCs, which were isolated and stimulated with M-CSF. Here, CSF1 is linked to rheumatoid arthritis.