Detection of prion infectivity in prion disease-affected human brain has also been studied by injection of human patient brain into susceptible animals. Transmission of sporadic, iatrogenic and variant forms of CJD has been demonstrated previously [3, 4, 16, 18, 25, 44, 47]. However, results of transmission experiments using familial prion disease brain have been more variable. Transmission experiments using human CNS tissue of familial prion disease patients have been done with ten of the 34 PrP mutations known to be associated with prion disease. In these experiments, seven mutations gave positive transmissions to rodents or primates. These included GSS-associated mutations (P102L, A117V, F198S) [3, 34, 44], FFI mutation (D178N with 129 MM)  and familial CJD mutations (D178N with 129VV, E200K, V210I, M232R) [3, 26, 44]. In contrast, no transmission was reported for mutations P105L, Y145X, or Y163X [27, 44]. It remains unclear whether these negative cases lacked any prion infectivity or were the result of low infectivity levels in the brain regions analyzed or the use of less sensitive host animals for the transmissions. Interestingly, two of these negative transmission cases involved patients with a mutation which created a stop codon resulting in PrP truncation (Y145X and Y163X), suggesting that PrP truncations might not generate spontaneous prion infectivity in vivo in humans.