PHR, a known weak estrogen [32,33] used as a pH indicator at a concentration of 15-45 μM in most tissue culture media, can bind to the ERα of MCF-7 human breast cancer cells at an affinity of 0.001% of E2 and was seen to reduce ERα-mediated growth stimulatory processes of exogenous estrogens [32]. The gene discussed is ESR1; the disease is breast cancer.