Creutzfeldt-Jakob's disease and bovine spongiform encephalitis (BSE) are both prion diseases, i.e., diseases caused by an abnormally folded isoform of cellular prion protein. A variant of Creutzfeldt-Jakob's disease can probably be transmitted from cattle with BSE to humans. To prevent spread of BSE, whole stocks of cattle are destroyed when symptoms of the disease appear. However, this is too late to prevent transmission during the about 5 years long incubation time. A method for presymptomatic diagnosis of BSE is clearly desirable. Miele and colleagues at Roslin Institute in Edinburgh present a potential molecular marker for prion diseases in the March issue of Nature Medicine. It is a dramatically decreased expression of a transcript called erythroid differentiation-related factor (EDRF). This change is detectable early in the course of the disease also in tissues outside the central nervous system, for instance blood.