 - The nucleoside analog 4-thio-2-deoxycytidine TdCyd is incorporated into DNA where it engages the active site of DNA methyltransferase I DNMT1, a maintenance methyltransferase that contributes to the hypermethylation and silencing of tumor suppressor genes. DNMT1 can become trapped in a covalent complex with DNA, thus depleting free enzyme and inhibiting the normal maintenance methylation of CpG sites, resulting in reactivation of tumor suppressor genes. 