The second sets into motion a runaway process that converts normal prions into a toxic form.

But the infectious prions fold differently than the normal prions.

The normal prion is a healthy brain protein that may play a role in nerve signaling.

No one has yet shown that a normal prion can be transformed into an abnormal prion and then cause disease.

Does a normal prion bind to an abnormal prion and then change its shape?

For example, how does the infectious prion alter the normal prion?

Scientists also know that normal prions must be present for the disease to take hold, which in this transgenic mouse could happen only in muscle.

They point especially to the fact that no one has been able to convert a normal human prion into a lethal one in a test tube.

Enzymes are added that digest normal prions, leaving only the abnormal misfolded ones.

But false positives can result when some normal prions are incompletely digested.