DNA methylation plays important roles in gene regulation and is highly relevant to human diseases and cancer. The most common types of DNA methylation include 5 methylcytosine (5mC), N4 methylcytosine (4mC) and N6 methyladenine (6mA). Compared to 5mC, which is the most studied DNA methylation, 6mA and 4mC are at lower levels and only detectable by highly sensitive technologies.