PRNP and nervous system disorder: Since the TgSprn24551 line of Sho Tg mice has a) no discernable alteration in the levels of PrPC (Figure 6b, c and Figure 7c), with PrPC being the obligate precursor to PrPSc, and b) has yet to show signs of spontaneous neurologic disease at ages up to 500 days (n  =  20), it was deemed suitable for prion challenge studies.