Hypermuscularity of Compact mouse is caused by a 12-bp deletion in the myostatin gene, but the molecular basis of decreased myostatin activity is unclear since the deletion does not affect the integrity of the growth factor domain. In the present work we show that the deletion causes misfolding and impaired secretion of myostatin precursor with concomitant decrease in myostatin activity. We suggest that some modifier genes that influence the expression of the Compact phenotype of myostatin mutant MstnCmpt-dl1Abc mice may exert their action through their role in the Unfolded Protein Response.