MIT computer scientist Manolis Kellis led the massive data crunching effort. (M. Scott Brauer)

Researchers have released the most complete mapping of the human epigenome to date, using massive data analysis to understand how and why certain genes turn off and on in human cells. Their work, published Wednesday in the journal Nature, could lead to new treatments for a multitude of diseases, including cancer.

But what the heck's an epigenome?

Over a decade ago, the efforts of the Human Genome Project gave us a complete road map of the genes that make up human DNA. But there's more to your genes than just which ones are present: The human genome is the musical score of human existence, but the human epigenome is the conductor. Without knowing what's going on with the epigenome, we can't really know how the music is going to sound.

So it's not hard to understand why the epigenome is so much trickier to pin down. Your DNA is essentially the same line of code in every single cell in your body, but not all of your cells perform the same jobs. Cells come together to build tissues as different from each other as muscle and bone, even though they all technically have the same genes inside them. The intricate and minute choices that your cells make about which genes to express are what we call the epigenome, and this can be influenced by basically anything: Where your mother lived while pregnant with you, what kind of food you're eating, and even what kinds of pollutants your grandparents were exposed to as children.

To create a better picture of these switches, researchers led by MIT professor Manolis Kellis analyzed a total of 150 billion genome sequencing reads. Using a specially developed algorithm, they crunched this massive data set to look for epigenomic changes in 111 different cell types.

“Different combinations of epigenetic marks characterize different regions of the genome, reflecting the specific functions that they play in each cell,” Kellis said in a statement. “By studying these combinations systematically, we can learn the language of the epigenome, and what it is telling us about both the activity and the function of each genomic region in each of the cell types.”

In one of the studies published Wednesday, the researchers tied specific cell changes to 58 different biological traits. Sometimes the epigenomic changes of a cell reveal possible clues about disease. For one example, Kellis told The Verge, they found mutations associated with Alzheimer's disease changing the genomic activity of immune cells -- not just brain cells. This was found in a mouse model, not a human, so it might not apply to us. But if it does, it could give researchers new areas to target when they try to treat the disease.