This article was reported and written in a collaboration with ProPublica, the nonprofit investigative journalism organization.

One of the world’s top breast cancer doctors failed to disclose millions of dollars in payments from drug and health care companies in recent years, omitting his financial ties from dozens of research articles in prestigious publications like The New England Journal of Medicine and The Lancet.

The researcher, Dr. José Baselga, a towering figure in the cancer world, is the chief medical officer at Memorial Sloan Kettering Cancer Center in New York. He has held board memberships or advisory roles with Roche and Bristol-Myers Squibb, among other corporations, has had a stake in start-ups testing cancer therapies, and played a key role in the development of breakthrough drugs that have revolutionized treatments for breast cancer.

According to an analysis by The New York Times and ProPublica, Dr. Baselga did not follow financial disclosure rules set by the American Association for Cancer Research when he was president of the group. He also left out payments he received from companies connected to cancer research in his articles published in the group’s journal, Cancer Discovery. At the same time, he has been one of the journal’s two editors in chief.