Various strategies have been used to disrupt the locus of the Prnp gene(which encodes PrP) to create different strains of Prnp knockout mice. Shown here are the structures of exon 3 in the Prnp genes of wild type and Zurich I strains of mice. The dotted line indicates the segment of Prnp DNA that has been deleted. Dark red, ORF; pink, noncoding region; and neo, neomycin phosphotransferase.