A summary of features found in PrP loci. (A) The human PrP locus. (B) The sheep PrP locus. (C) The mouse locus (Prnp ^a). (A–C) (1) The positions of the exons in each species. The scale is in kb. (2) The frequency of obseved CpG dinucleotides divided by frequency of expected CpG . CpG frequencies were calculated in a window length of 1000 base positions and an offset length of 100 bp. Note that an unusually high CpG frequency is found in the promoter region of each PrP gene. The mouse PrPgene has a second high peak in intron 2 owing to the IAP insertion. (3) Simple repeats at least 10 nucleotides in length.