So the researchers sought to determine which switches were active in more than 100 human cell types, an undertaking that required thousands of genome-wide experiments and the collection of huge amounts of data. For each tissue sample, they asked which regulators were turned on when different switches were turned on. That let them infer, in a sort of guilt-by-association way, whether a regulator controlled a switch. If the researchers always saw that when regulator X was turned on, switch Y was turned on, they could infer that regulator X controlled switch Y.

“Before, we did not have enough data to do that,” Dr. Lander said.

Dr. Lander explained the method the scientists used with an analogy to a Boston subway line that runs near his snow-enshrouded office in Cambridge. If you knew when service on the Red Line was disrupted and when various employees were late for work, you might be able to infer which employees lived on the Red Line, he said. Likewise, when a genetic circuit was shut down, certain genes would be turned off. That would indicate that those genes were connected, like the employees who were late to work when the Red Line shut down.

Finding the switches has already proved useful. Researchers had previously found DNA alterations associated with high blood pressure, for example, but the alterations were not in genes. Presumably they were in switches. The new research found that these switches were only active in the heart, and, it appears, in the heart muscle of the left ventricle.

In cancer, the disease may start with what Dr. John Stamatoyannopoulos of the University of Washington, one of the authors, called “a big bang of mutations” specific to a particular type of cell. That may help doctors figure out a mysterious sort of cancer, in which patients arrive with tumors all over the body but no one knows where the cancer started, making it difficult to find the best treatment. By looking at which switches are active they hope to figure out the tissue of origin.

But while the researchers are confident that their discoveries will be revelatory, they also see a long road ahead. They will find circuits, another author, Anshul Kundaje, an assistant professor of genetics at Stanford said. But, he added: “Making sense of them is a whole different story.”