USP39 and gastric cancer: Third, to test whether this interaction occurs endogenously in gastric cancer cells, we immunoprecipitated endogenous USP39 from MKN45 and HGC27 cell lysates using an anti-USP39 antibody and protein A/G agarose and detected the presence of endogenous RBM39 in the immunoprecipitates (Fig. 4, E and F).