This article discusses the identification and characterization of hematopoietic stem cells (HSCs) in mouse bone marrow. The study aims to isolate and analyze HSCs, which are rare cells that give rise to all blood cells. The researchers previously identified Lin-/Sca-1+/c-Kit+ cells as a potential HSC population, but further purification was needed. They then focused on CD34 as a new marker for purification. CD34 is a membrane protein involved in cell adhesion and is expressed in hematopoietic progenitor cells and endothelial cells. Using flow cytometry, the researchers analyzed CD34 expression in mouse bone marrow cells and found that most CD34-positive cells were Lin- (lineage-negative). Among Lin-/Sca-1+/c-Kit+ cells, over 90% were CD34-positive, but a small fraction was CD34-negative or weakly positive. The study used colony-forming unit-spleen (CFU-S) assays and hematopoietic reconstitution tests to determine the HSC activity of CD34-positive and negative cells. Results showed that most CFU-S-forming cells were CD34+/Lin-/Sca-1+/c-Kit+ cells, suggesting that these cells contain some differentiated progenitor cells. Additionally, the hematopoietic reconstitution ability of CD34+/Lin-/Sca-1+/c-Kit+ cells was examined, and it was found that these cells could maintain long-term hematopoiesis for over a year. The study also demonstrated that over 80% of CD34+/Lin-/Sca-1+/c-Kit+ cells could form mixed colonies, indicating a homogeneous population of HSCs. Furthermore, when individual cells were transplanted, they could regenerate hematopoiesis for over a year, proving that these cells are highly enriched in HSCs. The study concludes that CD34+/Lin-/Sca-1+/c-Kit+ cells in mouse bone marrow are a specific population of HSCs. However, the findings may conflict with previous results in humans, suggesting differences in CD34 expression between species. The researchers propose that human HSCs may exist in CD34-negative or weakly positive fractions. This has implications for clinical applications, as CD34-based cell enrichment is currently used in hematopoietic stem cell transplantation. The study demonstrates the potential of CD34+/Lin-/Sca-1+/c-Kit+ cells as a highly enriched HSC population, and the author is deemed qualified for a PhD in Medicine.This article discusses the identification and characterization of hematopoietic stem cells (HSCs) in mouse bone marrow. The study aims to isolate and analyze HSCs, which are rare cells that give rise to all blood cells. The researchers previously identified Lin-/Sca-1+/c-Kit+ cells as a potential HSC population, but further purification was needed. They then focused on CD34 as a new marker for purification. CD34 is a membrane protein involved in cell adhesion and is expressed in hematopoietic progenitor cells and endothelial cells. Using flow cytometry, the researchers analyzed CD34 expression in mouse bone marrow cells and found that most CD34-positive cells were Lin- (lineage-negative). Among Lin-/Sca-1+/c-Kit+ cells, over 90% were CD34-positive, but a small fraction was CD34-negative or weakly positive. The study used colony-forming unit-spleen (CFU-S) assays and hematopoietic reconstitution tests to determine the HSC activity of CD34-positive and negative cells. Results showed that most CFU-S-forming cells were CD34+/Lin-/Sca-1+/c-Kit+ cells, suggesting that these cells contain some differentiated progenitor cells. Additionally, the hematopoietic reconstitution ability of CD34+/Lin-/Sca-1+/c-Kit+ cells was examined, and it was found that these cells could maintain long-term hematopoiesis for over a year. The study also demonstrated that over 80% of CD34+/Lin-/Sca-1+/c-Kit+ cells could form mixed colonies, indicating a homogeneous population of HSCs. Furthermore, when individual cells were transplanted, they could regenerate hematopoiesis for over a year, proving that these cells are highly enriched in HSCs. The study concludes that CD34+/Lin-/Sca-1+/c-Kit+ cells in mouse bone marrow are a specific population of HSCs. However, the findings may conflict with previous results in humans, suggesting differences in CD34 expression between species. The researchers propose that human HSCs may exist in CD34-negative or weakly positive fractions. This has implications for clinical applications, as CD34-based cell enrichment is currently used in hematopoietic stem cell transplantation. The study demonstrates the potential of CD34+/Lin-/Sca-1+/c-Kit+ cells as a highly enriched HSC population, and the author is deemed qualified for a PhD in Medicine.