Young adult (6–8 week) tg66 mice were infected with brain homogenates derived from three human donors with different PRNP mutations (Y226X, Q227X, G131V). Each mouse was anesthetized with isoflurane and then intracerebrally injected in the left hemisphere with 30 μl of a 1% brain homogenate stock diluted in PBS. Alternatively, some mice were stereotactically microinjected in the striatum with 1 μl of 10% brain homogenate. This method was used to facilitate possible early detection of PrPSc replication at the site of the needle track, as was demonstrated in previous experiments at 3 to 7 days post-microinjection of mouse scrapie .