True. Chemical agents can cause mutations by ethylating guanine residues in DNA. When a chemical agent such as ethyl methanesulfonate (EMS) or ethyl nitrosourea (ENU) ethylates guanine residues, it can cause the formation of an ethyl adduct on the DNA strand. This ethyl adduct can lead to base-pair mispairing during DNA replication, which can cause mutations in the DNA sequence. The specific mutation that results from ethylation of guanine residues depends on the context of the adduct and the cellular repair mechanisms that are activated in response to the damage.