An orally bioavailable, nucleoside analog and DNA methyltransferase I (DNMT1) inhibitor, with potential DNA hypomethylating and antineoplastic activities. Upon administration, 5-aza-4'-thio-2'-deoxycytidine (Aza-TdC) gets incorporated into DNA, where it binds to the active site of DNMT1, a maintenance methyltransferase that contributes to the hypermethylation and silencing of tumor suppressor genes. The formation of covalent DNMT1-DNA complexes inhibits DNMT1, prevents DNA methylation of CpG sites, causes CpG demethylation, and results in the re-expression and re-activation of silenced tumor suppressor genes. This inhibits tumor cell proliferation. DNMT1, overactivated in tumor cells, plays a key role in tumor cell proliferation.