Iododeoxyuridine (5-Iodo-2'-deoxyuridine, IdU) is a synthetic thymidine nucleoside analog. IdU, like BrdU thymidine analog, is used to allow the detection of growing or proliferating cells in living tissues. During the S-phase of cell division, DNA replication occurs, and IdU can be incorporated into the newly synthesized DNA by substituting for naturally occurring thymidine.