The aim of the current study was to determine the expression profile of LDH-A and B in normal (MCF10A) as well as in endocrine-resistant (pII, MDA-MB-231, and YS2.5) and responsive (MCF7, and YS1.2) breast cancer cells and the effect of their knockdown on LDH activity, lactate production, proliferation and cell motility. The gene discussed is LDHA; the disease is breast cancer.