The sensitivity of tg66 mice to transmission of human sCJD and vCJD was reported previously [37, 38], and this sensitivity was emphasized in the present study by the transmission of infectivity from Y226X brain obtained from formalin-fixed tissue sections which were removed from microscope slides. Previous experiments have shown that prion infectivity is only partially sensitive to treatment with formalin , and can be obtained from formalin-fixed brain [2, 15, 32, 35]. In the case of Y226X, formalin-fixed brain was the only tissue available, even in the initial description of this patient . In our experiments with Y226X brain, positive transmission was seen in only four of the eight mice observed for > 590 days. Thus, there was approximately 1 ID50 in the 30 μl volume of 1% brain homogenate injected into these mice. This titer appeared to be at the threshold of our ability to detect transmission in the tg66 mouse system.