Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by pyruvate kinase deficiency ?
    ### Response:
    Pyruvate kinase deficiency is the most common inherited cause of nonspherocytic hemolytic anemia. More than 500 affected families have been identified, and studies suggest that the disorder may be underdiagnosed because mild cases may not be identified.  Pyruvate kinase deficiency is found in all ethnic groups.
    