The study by Al-Hajj et al. (2003) investigates the identification and characterization of tumorigenic breast cancer cells. Using a model where human breast cancer cells were transplanted into immunocompromised mice, the researchers found that only a small subset of cancer cells had the ability to form new tumors. These tumorigenic cells were identified as CD44+CD24−/lowLineage− and were present in 8 out of 9 patients' tumors. Even a small number of these cells (as few as 100) was sufficient to form tumors in mice, while larger numbers of other cancer cell types failed to do so. The tumorigenic cells could be passaged serially, generating new tumors each time, and they exhibited properties similar to stem cells, including self-renewal and the ability to give rise to diverse cell types. The identification of these tumorigenic cells opens up new avenues for understanding the biology of breast cancer and developing more effective therapies by targeting this critical population of cancer cells.The study by Al-Hajj et al. (2003) investigates the identification and characterization of tumorigenic breast cancer cells. Using a model where human breast cancer cells were transplanted into immunocompromised mice, the researchers found that only a small subset of cancer cells had the ability to form new tumors. These tumorigenic cells were identified as CD44+CD24−/lowLineage− and were present in 8 out of 9 patients' tumors. Even a small number of these cells (as few as 100) was sufficient to form tumors in mice, while larger numbers of other cancer cell types failed to do so. The tumorigenic cells could be passaged serially, generating new tumors each time, and they exhibited properties similar to stem cells, including self-renewal and the ability to give rise to diverse cell types. The identification of these tumorigenic cells opens up new avenues for understanding the biology of breast cancer and developing more effective therapies by targeting this critical population of cancer cells.