The aim of the current study was to investigate how differences in the culture media with respect to basal medium and hormones/growth factors affect transcriptional expression of breast cancer markers and mammary epithelial cell lineage regulatory transcription factors in reference breast cancer cell lines—luminal MCF7, HER2-enriched luminal SkBr3, and claudin-low MDA-MB-436. The gene discussed is ERBB2; the disease is breast cancer.