This review is aimed to provide the up-to-date knowledge on phosphofructokinase (PFK), a key enzyme of glycolysis, with special references to the recent advances of molecular biology of the enzyme. In human, three isozymes named M (muscle), L (liver) and P (platelet) are identified. Recently, mRNA and gene structures of these isozymes have been clarified. Deficiency of PFK-M is characterized by muscle weakness due to fuel crisis in exercising muscles. Up to now, gene defects of PFK deficient patients have been sought in 38 alleles from Japanese, Ashkenazi Jewish, Non-Ashkenazi Italian, French Canadian and Swiss patients and molecular heterogeneity has been elucidated. Down syndrome, in which trisomy of chromosome 21 is known provides us an interesting gene-dosage effect on PFK-L isozyme. Other various pathologic states affecting PFK activity in vivo are also reviewed briefly.