In summary, the detection of PrPSc by both immunoblot and IHC in 4 of 8 mice studied between 593 and 762 dpi supported the conclusion that transmission of prion disease occurred in this experiment using brain from the Y226X patient. The finding that only half the mice were positive, even after such a long observation period, suggested that the amount of infectivity transferred was near the end-point of the sensitivity of this transmission system.