5mC is a hotspot for mutation. (1) What is the chemical basis for the hotspot phenomenon? (2) And what base does 5mC typically change to? In looking at the frequency of all 16 possible dinucleotides in mammalian genomic DNA (AA, AC, AG, AT, CA, CC, etc), researchers long ago observed their frequencies were somewhat different from the expected 1/16. Modest deviations from 1/16 frequency can be explained by the fact that the G+C:A+T ratio in mammalian DNA is approximately 2:3. But the biggest discrepancy is that the CG dinucleotide frequency is much lower than 1/16. For example, in humans it is 5-fold lower. (3) Can you offer an explanation for this fact?