PRNP and variant Creutzfeldt-Jakob disease: Although homogenate substrate used for Surf-PMCA originated from tgHu mice (overexpressing 6 times the physiological level of human PrP) (46), survival time bioassays based on this transgenic line are limited by the long incubation period of vCJD (exceeding 500 d.p.inoc when undiluted) (45).