A search for differentially expressed genes was done by comparison of the expression profiles of FVB/N versus FVB/N Prnp-/- [10,30] mouse brains. To this aim, RNA samples were prepared by pooling RNAs from 5 brains of 6-week old mice of each genotype. After statistical analysis, two genes were found to be differentially expressed, including the Prnp one (Table 1). The relatively low log ratio observed for the variation in Prnp expression is explained by the fact that the knockout experiment was done in such a way that the gene remains expressed although at a lower level, around 2 to 3 fold, as observed by Northern blotting (data not shown), but that the resulting mRNA does not encode for PrP anymore [10].