Mammals express four PK isozymes: PKM1, PKM2 (the most prevalent in cancer cells), L and R. As to the two M isoforms, PKM1 is a constitutively active variant that is often expressed in differentiated tissues, while PKM2 exists as a low-activity dimer that converts to an active tetramer upon binding of F1,6P. The gene discussed is PKM; the disease is cancer.