5-Chloro-2'-deoxyuridine (5-Chlorodeoxyuridine) is a thymine analog. It can be used to study the potential of hypochlorous acid damage to DNA and DNA precursors.

The mammalian cells incorporate the analog into DNA when 5-Chloro-2’-deoxyuridine (ClDU) is placed into the tissue culture medium. It is observed that a 10 μM concentration of 5-Chloro-2’-deoxyuridine in the media does not alter cell division kinetics. Previously it has been shown that 5-Chloro-2’-deoxyuridine is metabolized and incorporated into DNA using antibodies that bind selectively to DNA containing halogenated bases. In this study, 5-Chloro-2’-deoxyuridine is more similar to BrdU in acting as a T analog. The toxicity of 5-Chloro-2’-deoxyuridine could in part be attributed to inhibition of thymidylate synthase [1].