Tumorigenesis and tumor progression are associated with dysfunction of the nuclear transport machinery at the level of import and export receptors (karyopherins). Recent studies have shown that the nuclear import factor karyopherin-α2 (KPNA2) is a novel prognostic marker for poor prognosis in human breast cancer. Based on the well-defined hallmarks of cancer progression, we performed a detailed in vitro characterization of the phenotypic effects caused by KPNA2 overexpression and KPNA2 silencing in benign and malignant human breast cells. KPNA2 overexpression clearly increased proliferation of MCF7 tumor cells and further led to a reduction of cell-matrix adhesion in benign MCF10A cells, whereas cell migration was significantly increased (P<0.0001) in both tumor models. Remarkably, these individual effects of KPNA2 overexpression on proliferation, cell-matrix adhesion and migration resulted in an increased colony spreading of benign MCF10A breast cells and malignant MCF7 tumor cells (P<0.001), which is a hallmark of cancer progression. Conversely, RNA interference-mediated KPNA2 silencing caused a complete inhibition of MCF7 tumor cell proliferation and migration (P<0.0001). In addition, in these experiments apoptosis was increased (P<0.05) and formation of tumor cell colonies was reduced (P<0.01). Thus, KPNA2 overexpression provoked increased aggressiveness of malignant MCF7 breast tumor cells and induced a shift in benign MCF10A breast cells toward a malignant breast cancer phenotype. In conclusion, we demonstrate for the first time in experimental tumor models that forced KPNA2 expression drives malignant features relevant for breast cancer progression, while its silencing is required for the remission of those progressive phenotypes. This study gives clear evidence that KPNA2 acts as a novel oncogenic factor in human breast cancer, in vitro.