DNA alkylating agents/cross-linking agents are molecules that alkylate or cross-link DNA. DNA alkylating agents/crosslinking agents may have mutagenic, pharmaceutical or other effects. Alkylation is the transfer of an alkyl group from one molecule to another. Alkylation agents are widely used in chemistry, as alkyl groups are probably the most common groups encountered in organic molecules. Use selective alkylation or adding parts with desired functional groups to the chain, especially if biological precursors are not normally available. Alkylation with just one carbon is called methylation. In medicine, alkylation of DNA is used in chemotherapy to destroy the DNA of cancer cells. Alkylation is accomplished with a class of drugs called alkylated antineoplastic agents. Cross-linking of DNA occurs when various foreign or endogenous reagents react with two different sites in the DNA. This can happen in the same strand (intra-strand crosslinking) or in opposite strands of DNA (interstrand crosslinking).