Expression of the human telomerase reverse transcriptase gene is modulated by quadruplex formation in its first exon due to DNA methylation.

DNA secondary structures and methylation are two well-known mechanisms that regulate gene expression. The catalytic subunit of telomerase, human telomerase reverse transcriptase (), is overexpressed in ∼90% of human cancers to maintain telomere length for cell immortalization. Binding of CCCTC-binding factor (CTCF) to the first exon of the gene can down-regulate its expression.