4-thio-2-deoxycytidine (TdCyd) is an orally bioavailable 4-thio modified 2-deoxycytidine analog, with potential antineoplastic activity. Upon administration of 4-thio-2-deoxycytidine (TdCyd), this cytidine analog is incorporated into DNA during replication and inhibits the activity of DNA methyltransferase 1 (DNMT1), which blocks DNA hypermethylation. This results in DNMT1 depletion, hypomethylation of DNA, and the reactivation of tumor suppressor genes that were silenced by hypermethylation; this results in antitumor activity and an inhibition of tumor cell proliferation. Data generated at Southern Research Institute and the NCI suggest a correlation between TdCyd activity in solid tumor xenograft models and decreased levels of DNMT1. TdCyd offers an improvement over current DNA methyltransferase inhibitors by virtue of a higher incorporation rate into DNA at lower levels of cytotoxicity; treatment with TdCyd is anticipated to result in the inhibition of tumor growth due to DNMT1 depletion at oral doses that are well tolerated in extended dosing schedules.