MFM P4 
Genetic Testing is Available
MFM is Myofibrillar Myopathy - it is dominant disease
MFM symptoms include exercise intolerance, tying up, pain, stiffness and tremors, heavy sweating, and in severe cases an inability to stand. Another symptom is very dark coffee colored urine. All of these signs often come after exercise and/or high stress.
MFM can be seen clearly on a muscle biopsy.
Horses that carry one copy of either P3 or P4 have a predisposition to develop MFM (also diagnosed as PSSM2). Horses with two copies of the variant (P3/P3 or P4/P4) have stronger symptoms and the symptoms show up when the horses are younger.
Treatment includes a high portein diet and a specific exercise plan.
P3 and P4 are used to signify MFM at this time.*
*Researchers at EquiSeq have identified semidominant alleles of two genes that causes MFM in Thoroughbreds and related breeds. The results are not yet published in a peer-reviewed academic journal. Prior to publication, the variants have been termed P3 and P4.
You can get your horse tested for P4 at Equiseq.com