Pyruvate kinase (PK) is an enzyme critical to the anaerobic glycolytic pathway of energy production in the erythrocyte. If red blood cells are deficient in PK they are unable to sustain normal cell metabolism and hence are destroyed prematurely. This deficiency manifests as an hemolytic anemia of variable severity with a strong regenerative response. Also associated with the disease in dogs but not cats is a progressive myelofibrosis and osteosclerosis of unknown etiology and this feature, along with liver failure, is the major cause of death in affected dogs.