In this study, we investigated the role(s) of a glutathione S-transferase pi (GSTP1)-mediated lactic acid signaling that impacts the pentose phosphate pathway and redox balance, especially under glucose deprivation, emphasizing role(s) of a Lac-GSTP1-G6PD axis in ER/PR positive or HER negative types of breast cancer. The gene discussed is ESR1; the disease is breast cancer.