Every cell in your body carries a set of instructions, called genes, that are like a recipe for making you who you are. Half of these genes came from your mom and half from your dad. That's why you might have curly, brown hair just like your mom's and a pointy chin just like your dad's.
Even though every one of your cells has the same set of genes, not every cell in your body is the same. A liver cell is very different from a muscle cell or from a cell in your eye.
How can this be? It's because each type of cell has different genes turned on or off, like a light switch. The more complicated the roles a cell must play, the more genes it has actively turned on.
Scientists have made great progress figuring out which genes are turned on in which cells. Last week the science magazine Nature published an amazing list that shows how many genes are active in 37 different types of body tissue.
The research was done by a team of scientists in Rockville, Maryland. They hope the information will someday help doctors to understand and treat diseases better.
The editors of Nature called the work "the first atlas of ourselves."