Skip to main content


Extensive research is taking place investigating many hereditary forms of exertional myopathy. The former inaccurate classification of all symptomatic horses lacking the PSSM1 GYS1 mutation under the “PSSM2” umbrella has caused a great deal of confusion. Under this designation, all forms of unexplained exertional myopathy (hereditary or environmental) have been bundled together.


For this reason, a clarification is necessary to limit the term “PSSM2” to only those animals which have biopsies similar to PSSM1 but which lack the GYS1 mutation for PSSM1 predisposition. Other variants (eg. P2, P3, P4, Px, P8, K1) are more appropriately classified under the category of Muscle Integrity Myopathy (MIM).


With this reclassification, it becomes clear why the MIM variants do not correlate with affected PSSM2 horses diagnosed by muscle biopsy - MIM horses do not have any defects in glycogen storage. Published studies are comparing different populations of horses. Furthermore, published studies include horses in the “normal group” if they have normal biopsies, even if these horses have clinical symptoms of muscle disease or are too young to yet see symptoms of muscle disease.


generatio’s research concentration is on the characterization of variants/variant combinations, symptoms, and responses to management strategies. The company also supports studbooks and breed organizations with tracking the frequency of variants in different breed populations and supporting their research programs.