Murine neuroblastoma N2a cells (ATCC line CCL 131) were grown in the Minimal Essential Medium supplemented with 10% FBS, penicillin and streptomycin and infected with 2% 22L brain homogenate as described previously [12] . Following infection, the amount of PrP Sc in 200 mg cell lysate aliquots of the N2a/22L cells was determined by PK digestion (1 mg/ml PK for 30 min at 37uC), SDS-PAGE on 12.5% Tris-tricine gels [37] and western blot analysis as previously described [12] .