PrPs from the various spongiform encephalopathies have been sequenced and found to differ, in some cases by very little, in some cases by quite a bit. For example, recent research has shown that the scrapie PrP protein differs from the BSE PrP protein at only seven amino acid loci, whereas the BSE PrP protein differs from the human CJD PrP at more than 30 loci. These differences explain the concept of strains and help explain why prions from one species might jump more easily into another species than another. It is difficult to find the terms to discuss prions -- for example, can we talk about mutants when there is no DNA?