We observed ARPP-19 was up-regulated in Herceptin resistance gastric cancer cells NCI-N87-HR and MKN45-HR. The forced expression of ARPP-19 promoted, whereas the silencing of ARPP-19 impaired Herceptin resistance of HER2-positive gastric cancer cells both in vitro and in vivo. Moreover, ARPP-19 significantly enhanced the sphere formation capacity and CD44 expression, CD44 was also a positive factor of Herceptin resistance in HER2-positive gastric cancer cells....Both the mRNA (Figure 1B) and protein (Figure 1C) levels of ARPP-19 were obviously up-regulated in NCI-N87-HR and MKN45-HR cells compared with NCI-N87 and MKN45 cells, respectively. Thereby, ARPP-19 was over-expressed in GC cells with acquired Herceptin resistance.