Resistance = arginine, R, at 171
Susceptibility = glutamine, Q, at 171
The other 2 amino acids have a less marked effect but are considered in the scoring scheme of clinical scrapie risk ( R ) where R1= resistant and R5= fully susceptible
The alleleles may delay but not prevent ataxia at 7y instead of 2y

How is this applied to flock improvement?

Genotyping of rams is by DNA-sequencing of a PCR-amplified fragment of wbc from EDTA blood, (avoid heparin which can interfere with PCR), is now provided by the Veterinary laboratory agency (VLA) and Scottish Agricultural Colleges (SAC).
Because susceptibility is a recessive condition a homozygote resistant ram will not sire susceptible lambs and they are more useful than ewes for reducing flock-deaths from Scrapie.
Scores are also used in lamb sales.
R4 and R5 rams with exceptional other traits should be backcrossed to R1 or R2 ewes if used.