In the case of mice expressing WT PrP at 3–4× levels, incubation times with the same RML prion isolate were 100 ± 17 days (Fischer et al, 1996) versus 89.1 ± 2.5 and 84.1 ± 2.6 days in the TgPrP(S3.F88W)-14 and TgPrP(S3.F88W)-35 lines with 3× expression, respectively (Tables1 and 2); these figures are in reasonable agreement given the margins of error in scoring the terminal phase of prion disease and could suggest that expression level is the key variable in arriving at incubation times < 100 days. The gene discussed is PRNP; the disease is prion disease.