Considering that upregulated PKM2 and LDHA, which are two crucial glycolytic enzymes, facilitate the growth advantage of tumor cells, and to determine the effect of ATRA on glycolysis in ER-positive and triple-negative cells, we examined the variation in the mRNA and protein expression of LDHA and PKM2, as well as PKM2 activity, in treated MCF-7 and MDA-MB-231 cells. The gene discussed is PKM; the disease is neoplasm.