PFKFB4 and endometriosis: In endometriosis, the expression of genes associated with the mitochondrial electron transport chain is reduced, whereas glycolytic enzymes such as HK2, pyruvate kinase, and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4 (PFKFB4) are upregulated [72,73,74].