The Sp1 transcription factor is crucial for CLDN3 and CLDN4 promoter activity, and the epigenetic status, including DNA methylation and histone H3 acetylation, in the critical region containing the Sp1 binding site also plays a role in the regulation of CLDN3 and CLDN4 expression in ovarian cancer cells [4,5]. The gene discussed is CLDN4; the disease is ovarian cancer.