The human body consists of more than 200 different types of cells, but all cells contain the same DNA code. How is it possible that the same code tells the various types of cells to be different?...Chemical attachments, which are called DNA methylation, can “turn off” parts of the DNA code that are not needed in a specific cell...Understanding how our surroundings can influence DNA methylation might also help us to better understand the mechanisms causing some diseases and hopefully get better at curing and preventing them. This article describes ADHD as one example.