PEAK1 and breast cancer: 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).