We further determine that PEAK1-expressing MSCs promote the emergence of p-Akthigh/p-γH2AXlow, MCL1high/p-γH2AXlow and GRP78high/VIMhigh subpopulations within HER2-positive breast cancer cells that display resistance to lapatinib and are capable of enhancing tumorigenesis in vitro and in vivo (Figs. 4–6 and 8). This evidence concerns the gene ERBB2 and breast cancer.