ANN ARBOR, Mich. - A dwarf mouse named Yoda has celebrated his fourth birthday, making him the oldest of his kind and far beyond 100 in human years, the University of Michigan Medical School says.

Yoda owes his longevity to genetic modifications that affected his pituitary and thyroid glands and reduced insulin production - and which left him a third smaller than an average mouse and very sensitive to cold.

On the other hand, at the human equivalent of about 136 years, Yoda is still mobile, sexually active and "looking good," said Dr. Richard A. Miller, associate director of research at the school's geriatrics center.