Methyl-piperidino-pyrazole (MPP) dihydrochloride is a highly selective antagonist for estrogen receptor alpha (ERα), with an affinity 200 times greater than for estrogen receptor beta (ERβ). It inhibits 17β-estradiol binding to ERα by inducing conformational changes that alter the receptor’s binding site and also increases the energy required for genistein’s interaction with ERα.