Methotrexate is often used as a chemotherapeutic agent to treat patients with leukemia. This drug is effective because it inhibits cells in which pa of the cell cycle

Methotrexate inhibits the synthesis of deoxythymidine by preventing the regeneration of THF by inhibiting the enzyme DHFR. Inhibiting the synthesis of deoxythymidylate prevents the cell from synthesizing its DNA. DNA synthesis occurs exclusively during S phase of the cell cycle. Ref:- KD Tripathi