Normal proliferating cells and tumour cells in particular express the pyruvate kinase isoenzyme type M2 (M2-PK, PKM2). The quaternary structure of M2-PK determines whether the glucose carbons are degraded to pyruvate and lactate with production of energy (tetrameric form) or channelled into synthetic processes, debranching from glycolytic intermediates such as nucleic acid, amino acid and phospholipid synthesis. The tetramer:dimer ratio of M2-PK is regulated by metabolic intermediates, such as fructose 1,6-P2 and direct interaction with different oncoproteins, such as pp60v-src kinase, HPV-16 E7 and A-Raf. The metabolic function of the interaction between M2-PK and the HERC1 oncoprotein remains unknown. Thus, M2-PK is a meeting point for different oncogenes and metabolism. In tumour cells, the dimeric form of M2-PK is predominant and has therefore been termed Tumour M2-PK. Tumour M2-PK is released from tumours into the blood and from gastrointestinal tumours also into the stool of tumour patients. The quantification of Tumour M2-PK in EDTA plasma and stool is a tool for early detection of tumours and therapy control.