ALDH1 expression level and ratio of tumor cells CD44+CD24low, CD326+CD44+CD24−, CD326−CD49f+, CSC of mesenchymal origin (CD44+CD24−) and epithelial tumor cells (CD227+CD44+CD24− and CD44+CD24−CD49f+), HSC, and CD309+ endothelial cells in tumors can act as personalized diagnostic markers, predictors of complications and the effectiveness of breast cancer treatment in further research. The gene discussed is ALDH1A1; the disease is breast cancer.