Just four letters—A, C, T, and G—make up an organism’s genetic code. Changing a single letter, or base, can lead to changes in protein structures and functions, impacting an organism’s traits. In addition, though, subtler changes can and do happen, involving modifications of the DNA bases themselves. The best-known example of this kind of change is a methylation of the base cytosine at the 5th position on its carbon ring (5mC). In eukaryotes, a less-well known modification involves adding a methyl group to base 6 of adenine (6mA).