<b>Methods</b>: We investigated the cytotoxic and metabolic effects of firsocostat in four breast cancer cell lines-MCF7 (luminal A HR<sup>+</sup>), SK-BR-3 (HER2-positive), MDA-MB-231 (triple-negative), and HCC1937 (triple-negative, BRCA1-mutated)-together with the non-tumorigenic MCF-10A line. The gene discussed is ERBB2; the disease is breast carcinoma.