A degenerative myopathy affecting only the supracoracoideus in the domestic turkey, with a greater incidence in white turkeys than in Broad Breasted Bronze, was first observed in 1967. The defect is variable in expressivity and penetrance and is apparently part of a polygenic system involving modifiers. The defect is found principally in turkeys of breeding age, and only to a very limited extent in birds under 24 weeks of age. The abnormality has a higher incidence in females. Muscle damage takes the form of a localized hemorrhage, edema, degeneration and necrosis of muscle leading to encapsulation. Ultimately there is liquefaction and absorption of necrotic tissue.