Cytarabine is an antimetabolic agent and a DNA polymerase with an IC50 of 16 nM for wild-type CCRF-CEM cells.Cytarabine is used mainly in the treatment of hematological malignancies such as acute myeloid leukemia (AML) and non-Hodgkin lymphoma. Because the arabinose sugar sterically hinders the rotation of the molecule within DNA, DNA replication ceases, specifically during the S phase of the cell cycle. Cytarabine also inhibits DNA polymerase, resulting in a decrease in DNA replication and repair. ^[1][2]

[1] http://en.wikipedia.org/wiki/Cytarabine

[2] Tobias SC et al. Mol Pharm. 2004 Mar-Apr;1(2):112-6.