The suggestion there is that both N, the normal protein, and P, the prion protein, are a product of a post-translational metabolic pathway that forks, leading to either N or P. The presence of P has a negative feedback effect on the fork yielding N, so that P causes less and less N to be made, and more and more P. (For the Creutzfeld-Jakob prion PrP, N corresponds to PrPC, and P corresponds to PrP