In a leap forward in understanding the basic science of one of the most lethal cancers, two groups of researchers have found mutations in most melanomas that are unlike any they have seen before in cancer. The changes are in regions that control genes, not in the genes themselves. The mutations are exactly the type caused by exposure to ultraviolet light, indicating they might be among the first DNA changes in a cell’s path to melanoma.

The discoveries, published online Thursday in two papers in the journal Science Express, do not immediately suggest new treatments or ways to prevent melanoma, researchers said. But the findings help explain how melanomas — and, possibly, other cancers — develop and what drives their growth, insights that may be critical to long-term efforts to develop ways to prevent or stop the cancer.

For years, cancer researchers have searched for mutations in genes, but this time, they looked for — and found — mutations in a region that regulates genes. They did it by examining the entire DNA of multiple tumors, studying not just genes but also what has been called the dark matter, the 99 percent of the DNA that includes regions that control genes.

“You could think of this as one glimmer in what has been called cancer’s dark matter,” said Dr. Levi A. Garraway of the Dana-Farber Cancer Institute and the Broad Institute of Harvard and M.I.T.