Washington ? When it comes to pain, people can be wimps, stoics or somewhere in between. Now scientists have found one reason — a variation in a single gene that shows stoics really can tolerate more pain.

The discovery by University of Michigan neuroscientists emphasizes the need to customize pain treatment — and might even allow doctors to soon try predicting which patients will respond to a certain kind of medication.

The new research shows how much you suffer is due at least partly to a gene that helps regulate how many natural painkillers, called endorphins, your body produces.

The gene produces an enzyme called COMT that metabolizes the brain chemical dopamine, which acts as a signal messenger between brain cells.

Everyone has two copies of this gene, one inherited from each parent — but they can inherit forms that differ by one amino acid. The COMT gene that contains the amino acid methionine, or met, is less active than if it contained the amino acid valine, or val.

Dr. Jon-Kar Zubieta put 29 healthy young adults into brain-imaging PET scanners. He injected their jaw muscles with enough salt water to make them really ache. Zubieta measured how their brain cells reacted while the volunteer victims rated, every 15 seconds, how much they hurt during the 20-minute pain cycle.

People who had two copies of the val-COMT gene were stoics. In contrast, people with two copies of the met-COMT gene suffered the most pain from the smallest saline injections — and had far less natural painkiller action.