POU5F1 and breast cancer: OCT4 expression is lost upon additional differentiation and maturation,35 associated with DNA methylation of OCT4.36 Additionally, in human teratoma cells (OCT4 positive control), the distal enhancer region of OCT4 was found to be completely unmethylated (0.0%), whereas in HeLa and MCF‐7 cells, the promoter region was highly methylated (92.3% and 98.7%, respectively), suggesting that expression of OCT4 was completely silenced in both of these cell lines.25 Loss of OCT4 in breast cancer may predominantly correlate with epigenetic mechanisms, such as DNA methylation.