This enzyme has been chosen for different reasons: its modulation permits to control both N- and O-GlcNAc; it is downstream to the salvage pathway; it is an unfavorable marker for breast cancer;11 mouse and human cells, in which PGM3 activity is reduced, show a decrease in HBP flux associated with the reduction of complex N-glycans and of O-GlcNAc level12–14. The gene discussed is PGM3; the disease is breast cancer.