In late 1989, Mr. Tilousi’s uncle Rex Tilousi approached John Martin, an Arizona State University anthropologist who had gained the tribe’s trust, to ask if he knew a doctor who could help. “I asked him, ‘How can we prevent this from spreading?’ ” the elder Mr. Tilousi recalled.

Professor Martin approached Dr. Markow. A link had recently been reported between a genetic variant and the high rate of diabetes among Pima Indians. If a similar link was found among the Havasupai, it might point to an important risk factor.

The two professors received money from the university to study diabetes in the tribe. Dr. Markow was interested in schizophrenia research as well, and in the summer of 1990, with a grant from the National Alliance for Research on Schizophrenia and Depression, she and her graduate students began collecting blood samples in Supai. Women here remember being happy to see her in those days, an athletic figure who talked to them about how to be more healthy. Working out of the health clinic in the center of the village, Dr. Markow recruited tribe members to ask others to give blood. To the Havasuapi, blood has deep spiritual meaning.

“I went and told people, if they have their blood taken, it would help them,” said Floranda Uqualla, 46, whose parents and grandparents suffered from diabetes. “And we might get a cure so that our people won’t have to leave our canyon.” Roughly 100 tribe members who gave blood from 1990 to 1994 signed a broad consent that said the research was to “study the causes of behavioral/medical disorders.”

The consent form was purposely simple, Dr. Markow said, given that English was a second language for many Havasupai, and few of the tribe’s 650 members had graduated from high school. They were always given the opportunity to ask questions, she said, and students were also instructed to explain the project and get written and verbal consent from donors.

Dr. Markow examined several genes that were thought to have medical relevance, including for schizophrenia, metabolic disorders and alcoholism, she said, but found little to pursue. The Havasupai did not, it turned out, share the gene variant linked to diabetes in the Pima.

But a few years later, a graduate student using new technology came up with a way to discern variations in the Havasupai DNA, which was stored in a university freezer, and he wrote a dissertation based on his research.