Disruption of the Prnp gene. Exons are shown in light blue and the coding region (entirely contained in exon 3) is shown in dark blue. Part of the coding region in exon 3 is replaced by the neomycin phosphotransferase gene (neo, yellow) in a disruption construct containing exon 3 and about 5 kb of Prnp gene sequences upstream and downstream of exon 3. The disruption construct recombines with one of the Prnp genes in mouse ES cells (embryonic stem cells) under selection by the aminoglycoside drug G418, and the recombination event is verified by Southern blot hybridization. ES cells with the disrupted Prnp gene are used to create heterozygous mice (Prnp+/o) which are further bred to give homozygous mice (Prnpo/o).