Downregulation of YAP1 results in a weaker tamoxifen response in the T47D cell line. T47D cells were transfected with siCtr, siYAP1 #7 or siYAP1 #8 and 48 h later medium was changed to phenol red-free DMEM supplemented with 5% charcoal stripped serum. Twenty-four hours later, the indicated treatment was added and after 4 days, cells were analysed for (a) YAP1, cyclin D1 and cyclin A2 protein expression by western blot (b) viability by WST-1 assay. Treatment of T47D results in a significant tamoxifen response for siCtr and siYAP1 #7 at 10-7 M 4-OH-tam whereas siYAP1 #8 was not significantly affected until 10-6 M 4-OH-tam. SiCtr cells respond significantly better to increasing concentrations of 4-OH-tam (**p = 0.006). No significant changes in response was observed when increasing 4-OH-tam in siYAP1 downregulated cells (siYAP1 #7, p = 0.09 and siYAP1 #8, p = 0.10). All values were related to EtOH treated siCtr cells. (c) T47D cells were transfected with siCtr, siYAP1 #7 or siYAP1 #8 followed by transfection with pERE-luc and pRL-TK as internal control. The mean value ± SD is indicated and fold decrease upon tamoxifen treatment (compared to E2 activation) is shown below the graph. P-values were calculated using paired student’s t-test (two-sided). *p < 0.05, **p < 0.01, ***p < 0.001. 4-OH-tam = 4-hydroxi-tamoxifen, E2 = 17β-estradiol, n.s. = no significance.