The initial step in chemical carcinogenesis is characterized by attachment of the chemical to DNA to produce DNA adducts. The chemicals can alter the structure of the DNA and in turn, the biological processing of the DNA by cellular proteins governing replication, transcription and repair. If not repaired or repaired incorrectly, these modifications may eventually lead to mutations and ultimately cancer, especially if the adduct is located in an oncogene or tumor suppressor gene. Thus, different DNA adducts can affect cellular responses to DNA damage.