A major challenge for researchers using Hematopoietic Stem Cells (HSCs) is their identification and isolation from larger pools of cells. Its is estimated that Hematopoietic Stem Cells represent approximately 1 in 10,000 cells of the bone marrow and 1 in 100,000 cells in the blood. In addition, Hematopoietic Stem Cells are morphologically very similar to white blood cells. Thus, techniques designed for isolation and identification of HSCs are often dependent on the detection of cell surface markers and cluster of differentiation antigens. HSCs are commonly characterized by the absence of lineage-specific marker expression. Human Hematopoietic Stem Cells are determined as CD34+, CD59+, CD90/Thy1+, CD38low/-, c-Kit-/low, and Lin-. Mouse Hematopoietic Stem Cells are considered CD34low/-, SCA-1+, CD90/Thy1+/low, CD38+, c-Kit+, and Lin-. Detecting the expression of these marker panels allows separation of specific cell populations via techniques like fluorescence-activated cell sorting (FACS).