This review by Prat and Perou provides an in-depth analysis of the molecular portraits of breast cancer, focusing on the Claudin-low subtype. The authors highlight the importance of global gene expression analyses in understanding the heterogeneity of breast cancer and its impact on treatment responses and patient outcomes. They discuss the five intrinsic subtypes of breast cancer (Luminal A, Luminal B, HER2-enriched, Claudin-low, Basal-like) and the Normal Breast-like group, emphasizing the unique features of the Claudin-low subtype, which is enriched for mesenchymal and stem cell characteristics. The review also explores the clinical-pathological characteristics and treatment sensitivity of these subtypes, including the role of standard clinical-pathological markers and the potential of the stem cell hypothesis in understanding the biological complexity of breast cancer. Additionally, the authors discuss the development of biomarkers and therapeutic strategies based on these molecular subtypes, emphasizing the need for further research to improve patient outcomes.This review by Prat and Perou provides an in-depth analysis of the molecular portraits of breast cancer, focusing on the Claudin-low subtype. The authors highlight the importance of global gene expression analyses in understanding the heterogeneity of breast cancer and its impact on treatment responses and patient outcomes. They discuss the five intrinsic subtypes of breast cancer (Luminal A, Luminal B, HER2-enriched, Claudin-low, Basal-like) and the Normal Breast-like group, emphasizing the unique features of the Claudin-low subtype, which is enriched for mesenchymal and stem cell characteristics. The review also explores the clinical-pathological characteristics and treatment sensitivity of these subtypes, including the role of standard clinical-pathological markers and the potential of the stem cell hypothesis in understanding the biological complexity of breast cancer. Additionally, the authors discuss the development of biomarkers and therapeutic strategies based on these molecular subtypes, emphasizing the need for further research to improve patient outcomes.