PKM and cancer: The four pyruvate kinase isoforms are (1) PKL which is primarily expressed in the liver and kidneys, (2) PKR which is exclusively expressed in red blood cells, (3) PKM1 which is highly expressed in differentiated tissues with high energetic demands, and (4) PKM2 which is highly expressed in undifferentiated tissues as well as rapidly proliferating tissues including cancer (Jurica et al., 1998).