BUB1 and osteosarcoma: In order to study the effect of microgravity on cell proliferation, mammalian osteosarcoma cells and osteoblasts were kept under simulated microgravity in a rotating wall vessel (2D-RWVS) bioreactor, and then the changes in cell proliferation, spindle structure, expression of MAD2 or BUB1, and effect of MAD2 or BUB1 on the inhibition of cell proliferation are investigated.