Purpose The present study was undertaken to examine the amplification and expression status of Cks1 in breast cancer and its significance.Methods The amplification and expression status of Cks1 gene was examined by FISH,Real-time PCR and Immunohistochemistry,respectively.RNA interference was used to detect the effects of Cks1 on migration,invasion,cell cycle progress and apoptosis of breast cancer cells.Results Cks1 gene amplification was highly correlated with protein overexpression.Overexpression of Cks1 was strongly associated with lymph node metastasis and poor prognosisr.Knockdown of Cks1 expression by RNA interference inhibited the cell cycle progress,migration and invasion ability of breast cancer cells.Moreover,overexpression of Cks1 inhibited apoptosis of breast cancer cells through MEK-Erk pathway.Conclusion These data suggest that Cks1 is an oncogene in the 1q21 amplicon and may play an important role for breast cancer development.