To study the molecular mechanism by which MM-121 overcomes trastuzumab resistance and enhances trastuzumab’s efficacy on inhibition of cell proliferation and/or survival in the studied cell lines, we considered the mechanism of action of trastuzumab inducing cell cycle G1 arrest [4,31,32] and thus investigated the combinatorial effects of MM-121 and trastuzumab on the expression levels of several critical molecules participating in G1-S transition and cell cycle progression in erbB2+ breast cancer cell lines. This evidence concerns the gene ERBB2 and breast cancer.