O⁶-alkylguanine is an alkylating agent that is a chemical mutagen - a substance than can cause mutations in DNA. DNA that has been alkylated with O⁶-alkylguanine is mutagenic because the modified base has a very high probability of pairing with thymine when the modified strand replicates (Figure 25.11). Thus, alkylation of a DNA-guanine stimulates a GC ---> AT transition.

The enzyme O⁶-alkylguanine alkyltransferase repairs DNA damaged by 6-alkylguanine.