Pyruvate kinase (PK) is a key glycolytic enzyme and has two structural genes; the L/R-gene encodes the L- and R-type PK, whereas the M-gene encodes the M1- and M2-type isozymes. The isozyme switches from the M2 to the R-type during erythroid differentiation, and recent results showed that the switching was achieved by activation of the R-PK promoter activity and the involvement of erythroid-specific transcription factors has been demonstrated. Glycolysis is a major energy source for red cells, therefore, PK deficiency results in hemolysis. PK deficiency is the most common glycolytic enzyme defect associated with hereditary hemolytic anemia, and inherited in an autosomal recessive manner. To date, 46 gene mutations have been identified, and molecular approach might be helpful for diagnosis of PK deficiency, particularly among transfusion-dependent subjects or infantile cases.