To determine whether the decrease in KLF4 expression is due to the increase in methylation level, we investigated the methylation status of the KLF4 promoter in breast cancer tissues, and found that the methylation level of specific CpG site (−148 bp) in the KLF4 promoter played a critical role in the regulation of KLF4 expression. The gene discussed is KLF4; the disease is breast cancer.