Hematopoietic stem cells (HSCs) are multipotent stem cells that give rise to blood cell types of the myeloid and lymphoid lineages. Hematopoietic stem cells are formed from the mesoderm during embryogenesis and are deposited in specific hematopoietic sites within the embryo. In adults, HSCs are found in the bone marrow and peripheral blood. Other sources of HSCs include the placenta and umbilical cord blood. HSCc are currently the only type of stem cell used therapeutically HSC transplants are commonly used to treat cancers and other blood and immune system disorders. Recent animal studies indicate that HSCs may have the ability to transdifferentiate, forming certain types of muscle, blood vessel, and bone cells. HSCs closely resemble other blood and bone marrow cells and only some HSCs possess long-term replicating ability, rendering detection of optimal stem cells by morphology alone impossible.