This study identifies a distinct population of breast cancer cells, termed tumorigenic or cancer-initiating cells, which have the ability to form new tumors. These cells are characterized by the expression of CD44⁺CD24⁻/lowLineage⁻ markers and can be isolated from breast cancer samples. The tumorigenic cells are capable of self-renewal and can generate new tumors containing additional tumorigenic cells as well as phenotypically diverse non-tumorigenic cells. The study demonstrates that only a small fraction of breast cancer cells are tumorigenic, and that these cells can be serially passaged in mice, maintaining their tumorigenic potential. The identification of these cells provides a model for understanding the biology of breast tumors and may lead to the development of more effective therapies targeting these cells. The study also highlights the importance of identifying tumorigenic cells for understanding the mechanisms of tumor progression and for developing targeted therapies. The findings suggest that breast cancer tumors consist of a heterogeneous population of cells, with a small subset of cells capable of initiating and maintaining tumor growth. These tumorigenic cells may be responsible for the development of metastatic disease and resistance to therapy. The study also shows that tumorigenic cells are more resistant to chemotherapy than non-tumorigenic cells, which may explain their ability to survive and regenerate tumors after treatment. The identification of tumorigenic cells could lead to the development of new diagnostic tools and therapies for breast cancer.This study identifies a distinct population of breast cancer cells, termed tumorigenic or cancer-initiating cells, which have the ability to form new tumors. These cells are characterized by the expression of CD44⁺CD24⁻/lowLineage⁻ markers and can be isolated from breast cancer samples. The tumorigenic cells are capable of self-renewal and can generate new tumors containing additional tumorigenic cells as well as phenotypically diverse non-tumorigenic cells. The study demonstrates that only a small fraction of breast cancer cells are tumorigenic, and that these cells can be serially passaged in mice, maintaining their tumorigenic potential. The identification of these cells provides a model for understanding the biology of breast tumors and may lead to the development of more effective therapies targeting these cells. The study also highlights the importance of identifying tumorigenic cells for understanding the mechanisms of tumor progression and for developing targeted therapies. The findings suggest that breast cancer tumors consist of a heterogeneous population of cells, with a small subset of cells capable of initiating and maintaining tumor growth. These tumorigenic cells may be responsible for the development of metastatic disease and resistance to therapy. The study also shows that tumorigenic cells are more resistant to chemotherapy than non-tumorigenic cells, which may explain their ability to survive and regenerate tumors after treatment. The identification of tumorigenic cells could lead to the development of new diagnostic tools and therapies for breast cancer.