6-gingerol inhibits tumorsphere formation. (A) Suppression in sphere forming ability of MDA-MB-231 and MCF-7 cells by 6-gingerol treatment. Representative photographs are presented (scale bar, 15 μm). Cells were incubated with sphere forming media for 14 days with or without 200 µM 6-gingerol and 20 µM PFT-α. (B) qPCR analysis of expression of cancer stem cell marker genes. The representative expression of the SOX2, OCT4, and NANOG mRNA is shown; Cp values were then normalized to GAPDH mRNA. Controls are set to 100. *** p < 0.001 (ANOVA test). # p < 0.001 vs. control. (C) qPCR analysis of expression of p53 and intrinsic apoptosis genes. The representative expression of the TP53, BAX, and BCL-2 mRNA is shown; Cp values were then normalized to GAPDH mRNA. Controls are set to 100. *** p < 0.001 (ANOVA test). # p < 0.001 vs. control.