STK33 overexpression restores the growth of mutant KRAS-dependent breast tumors treated with HSP90 inhibitors. (a) Tumor formation on chicken CAM of mutant KRAS-dependent MDA-MB-231 breast cancer cells stably transduced with Flag-tagged STK33 or empty vector (EV) and treated with 5 µM 17-AAG or 1 µM PU-H71 for 48 h. Representative photographs of tumors and tumor areas (error bars represent mean ± SEM of four to six tumors) are shown. NS, not significant. Bar, 1.5 mm. (b) IHC analysis of CAM tumors shown in a. Representative photographs of tissue sections and the proportion of TUNEL-positive cells (error bars represent mean ± SEM of four microscopic fields with 600 cells) are shown. EV, empty vector. Insets show details of the corresponding photographs. Bar: 125 µm; inset, 25 µm.