 A single nucleotide base, a single letter in the DNA, could be damaged. In a process known as base excision repair, a suite of proteins recognize the trouble, snip out the problematic base, and sometimes also a bit of DNA around it, and then copy the correct sequence from the other strand. There's also a proofreading system called DNA mismatch repair to detect and correct mistakes that happen during replication, such as an addition or deletion or the wrong base being put in. But rather than listing all the different types of DNA damage and how they get repaired, let's focus on a couple that were the topic of a recent paper about DNA damage and aging.