In order to analyse the impact of ATM kinase on formation of cells with a stem-like phenotype in breast cancer, we choose as cancer model system: 1) mammospheres derived from luminal estrogen receptor-positive HER2-low breast cancer cell line (MCF7); 2) mammospheres derived from luminal estrogen receptor-positive HER2-overexpressing breast cancer cell line (MCF7-HER2). The gene discussed is ERBB2; the disease is breast cancer.