PKM and cancer: In humans, four pyruvate kinase isoforms can be found: (1) PKL, which is primarily expressed in the liver and kidneys; (2) PKR, which is exclusively expressed in red blood cells; (3) PKM1 (pyruvate kinase isoenzyme type M1), which is highly expressed in differentiated tissues with high energetic demands; and (4) PKM2 (pyruvate kinase isoenzyme type M2), which is highly expressed in undifferentiated tissues as well as in rapidly proliferating tissues including cancer [6].