RER
RER (PX*) is Recurrent Exertional Rhabdomyolysis - it is a dominant disease 
Genetic Testing is Available
RER shows up during or after exercise - horses are reluctant to move, experience pain, stiffness, and tremors, and sweat profusely. Severe episodes cause muscle damage, sometimes an inability to stand, and death can occur. Coffee colored urine indicates a severe episode. RER horses usually have a very excitable temperament and do not handle stress well.
A specific diet (including added protein and the elimination of suger) and exercise routine can help manage symptoms.
RER episodes will trigger high levels of serum CK and AST (values over 10,000, where the baseline value is under 500) and coffee-colored urine,
*Equiseq has developed a genetic test to help diagnose RER in horses.