This review discusses the molecular portraits of breast cancer, focusing on the Claudin-low subtype, which is characterized by mesenchymal and stem cell features. Breast cancer is a heterogeneous disease with five intrinsic subtypes: Luminal A, Luminal B, HER2-enriched, Claudin-low, and Basal-like, along with a Normal Breast-like group. The Claudin-low subtype is enriched for stem cell and mesenchymal features and is associated with poor prognosis, being ER-negative, PR-negative, and HER2-negative (triple-negative). Claudin-low tumors show intermediate response to neoadjuvant chemotherapy compared to Basal-like and Luminal tumors. They are enriched with genes linked to mammary stem cells, a Core EMT signature, and features of tumor-initiating cells. The review also discusses the potential developmental origin of each intrinsic subtype and the role of stem cell-like features in breast cancer biology. Claudin-low tumors are characterized by high mesenchymal features and low luminal/epithelial differentiation. They show high expression of vimentin and N-cadherin, and several transcriptional repressors of E-cadherin. Claudin-low tumors are also enriched with immune-related genes and show high immune cell infiltration. Claudin-low cell lines are enriched with stem cell-like features and have high expression of ALDH1. These cell lines are similar to Claudin-low tumors in gene expression and are enriched with stem cell-like features. Claudin-low tumors are also associated with BRCA1 dysfunction and have a high incidence of BRCA1 methylation. The review also discusses the clinical characteristics of the Claudin-low and other intrinsic subtypes, including their prognosis, response to treatment, and relationship with other subtypes. Claudin-low tumors are the least frequent subtype and are mostly high-grade, ER-negative, PR-negative, and HER2-negative (triple-negative) tumors. They show poor prognosis compared to Luminal A tumors but similar prognosis to other poor prognosis subtypes. Claudin-low tumors are also associated with metaplastic and medullary carcinomas, which are poorly differentiated triple-negative tumors. The review highlights the importance of molecular subtyping in breast cancer and the need for further research to improve treatment outcomes for patients with breast cancer.This review discusses the molecular portraits of breast cancer, focusing on the Claudin-low subtype, which is characterized by mesenchymal and stem cell features. Breast cancer is a heterogeneous disease with five intrinsic subtypes: Luminal A, Luminal B, HER2-enriched, Claudin-low, and Basal-like, along with a Normal Breast-like group. The Claudin-low subtype is enriched for stem cell and mesenchymal features and is associated with poor prognosis, being ER-negative, PR-negative, and HER2-negative (triple-negative). Claudin-low tumors show intermediate response to neoadjuvant chemotherapy compared to Basal-like and Luminal tumors. They are enriched with genes linked to mammary stem cells, a Core EMT signature, and features of tumor-initiating cells. The review also discusses the potential developmental origin of each intrinsic subtype and the role of stem cell-like features in breast cancer biology. Claudin-low tumors are characterized by high mesenchymal features and low luminal/epithelial differentiation. They show high expression of vimentin and N-cadherin, and several transcriptional repressors of E-cadherin. Claudin-low tumors are also enriched with immune-related genes and show high immune cell infiltration. Claudin-low cell lines are enriched with stem cell-like features and have high expression of ALDH1. These cell lines are similar to Claudin-low tumors in gene expression and are enriched with stem cell-like features. Claudin-low tumors are also associated with BRCA1 dysfunction and have a high incidence of BRCA1 methylation. The review also discusses the clinical characteristics of the Claudin-low and other intrinsic subtypes, including their prognosis, response to treatment, and relationship with other subtypes. Claudin-low tumors are the least frequent subtype and are mostly high-grade, ER-negative, PR-negative, and HER2-negative (triple-negative) tumors. They show poor prognosis compared to Luminal A tumors but similar prognosis to other poor prognosis subtypes. Claudin-low tumors are also associated with metaplastic and medullary carcinomas, which are poorly differentiated triple-negative tumors. The review highlights the importance of molecular subtyping in breast cancer and the need for further research to improve treatment outcomes for patients with breast cancer.