PQ is the unknown muscle issue 
No Genetic Testing is Available at this time
There are still some horses who exhibit classic PSSM issues but test clear for the known variants to date. Researchers are still putting together the puzzle of genetics to figure out as many genes as possible that are affecting horses.
PQ is the code we use for horses that have tested negative to all PSSM so far but are still symptomatic.
We will update here as info comes available.