Halogenated nucleosides are modified nucleosides that contain halogen atoms, such as fluorine, chlorine, bromine, or iodine. These compounds are important in biochemistry and molecular biology because they can interfere with DNA and RNA synthesis, making them useful in research and therapeutic applications, particularly in antiviral and anticancer treatments. These nucleosides can mimic natural nucleosides, allowing them to be incorporated into nucleic acids during replication. Their presence can lead to mutations or inhibit the replication of viruses and cancer cells, making halogenated nucleosides valuable tools in drug development and the study of genetic processes.