Furthermore and after taking into account our previously mentioned recent data showing that mAb 4C5 inhibits MDAMB453 human breast cancer cell invasion in vitro by disrupting the association of cell surface HSP90 with HER-2,[14] in this work we examined: a) the effect of mAb 4C5 on MMP2 and MMP9 secretion and activation b) the ability of this antibody to disrupt the interaction of extracellular HSP90 with MMP2 and/or MMP9 and c) the capacity of mAb 4C5 to inhibit in vivo, the formation of metastatic deposits of MDAMB453 cells in the lungs of SCID mice. This evidence concerns the gene MMP9 and breast cancer.