Our data show that geminin controls cytokinesis in human cells, preciously abscission, that geminin is a genuine breast cancer oncogene that induces tetraploidy/aneuploidy when overexpressed by inhibiting AurB, that this mechanism contributes to the induction of aneuploid, aggressive and metastatic breast tumors, and that therapeutic targeting of geminin might be pursued to inhibit breast cancer metastasis. Here, GMNN is linked to breast neoplasm.