For years, Terry Smith knew that he was distantly related to Abraham Lincoln. But he didn't know about the illness that ran in his family -- what relatives called "Lincoln's Disease."

Now, researchers at the University of Minnesota have uncovered a genetic secret that has plagued the Lincoln family for at least 11 generations: a mutation that causes a form of ataxia, a crippling neurological disease.

The discovery could shed new light on the causes of a number of similar diseases, said Laura Ranum, a university geneticist who led the research project.

But on a historic level, she said, it also raises intriguing new questions about the 16th president, whose health has long been a topic of speculation.

Ranum, who has studied the Lincoln family since 1992, doesn't know whether the president carried the defective gene.

But through genetic detective work, she and her colleagues studied 300 distant cousins of the president (he has no living descendents himself; the last, Robert Todd Lincoln Beckwith, died in 1985). They found that about a third have ataxia, including Smith, of Manteca, Calif.