Sca-1, or Stem Cell Antigen-1, is a glycosyl phosphatidylinositol-anchored cell surface protein (GPI-AP) expressed in mice. It is encoded by the murine gene, Ly6a, and may also be known as Lymphocyte antigen 6A-2/6E-1, Ly6, or T-cell-activating protein. This 18-kDa protein is a well-known marker for mesenchymal and hematopoietic stem cells. Its expression has also been observed in some cancer types. A human homolog of Sca-1 has yet to be discovered. Antibodies against Sca-1 are available from a variety of sources.