WASHINGTON, D.C. ? Scientists mapping genetics of two of the nation’s leading cancers have found almost 200 mutated genes in breast and colorectal tumors, many of them never before suspected of helping cancer form and spread.

Doctors have long known it takes a cascade of genetic flaws to trigger any of the myriad types of cancer. Which genes are working improperly also determines if a malignancy is especially aggressive, and even whether a particular treatment is likely to work.

Finding those genes-run-amok could lead to better cancer treatments and even ways to prevent tumors. But scientists have found only a fraction of them.

On Thursday, scientists at Johns Hopkins University reported the first big success: They examined more than 13,000 genes in 11 breast tumors and 11 colorectal cancers, removed from patients during surgery. They found 189 mutated ones that seem to play a role in those two types of cancer.