People on Deer Isle have a special interest in a rare disease called SPG4. Dozens of them carry the mutant gene that make them susceptible, although worldwide it strikes only two to four in 100,000.

And researchers keep looking for cases of another rare disease, Alstrom Syndrome, with a cluster of victims in the nearby Canadian Maritime Provinces.

SPG4, which stands for spastic paraplegia gait, affects walking or “gait.” It came to Deer Isle with an early settler from France. It is understood to affect only members and descendants of the Hardy or Haskell families. Children of carriers have a 50 percent chance of inheriting the mutation. It doesn’t shorten life, but it can weaken leg muscles and cause loss of balance.

Dr. Robert Beekman of Ellsworth, a specialist in the disease, relates its concentration on Deer Isle to the long isolation of the community there. He consults with Deer Isles residents who have the disease or fear they may get it.

Alstrom Syndrome is far more serious. It can strike in infancy and often leads to obesity, progressive loss of sight and hearing, heart problems and often early death. Jan D. Marshall, a genetics researcher at Jackson Laboratory, a specialist in the disease, knows of three cases in New Brunswick and five or six in Nova Scotia. A suspected case in Maine turned out to be something else, but the Acadian population in Maine and the Maritime Provinces is considered vulnerable. She says 657 cases in 48 countries have been reported.

Both are under close study by specialists, but no preventive or cure is yet in sight for either one. Both are so rare that the big genetic companies see no profitability in starting expensive research into their causes and possible cures.

Both Dr. Beekman and Ms. Marshall foresee a possibility of progress in the perhaps distant future. In the meantime, they direct interested people to websites and organizations that bring together patients and researchers. And a Jackson Lab colleague of Ms. Marshall has bred two Alstrom mouse models that they hope can lead to an understanding of the functioning of the Alstrom gene mutation as a first step toward dealing with the disease in humans.

In the meantime, specialists in such rare diseases hope that Gov. John Baldacci will join other states in designating Feb. 28 as Rare Disease Day to spread awareness and provide information and hope.