Sca-1 is an 18 kDa phosphatidylinositol-anchored protein that is a member of the lymphocyte antigen 6 (Ly-6) family. Sca-1 is encoded by the strain-specific Ly-6 E/A allelic gene. Its expression on multipotent hematopoietic stem cells (HSC) has been used as a marker of HSCs in mice of both Ly-6 haplotypes. Sca-1-positive HSCs can be found in the adult bone marrow, fetal liver, and mobilized peripheral blood and spleen in the adult animal. However, Sca-1 has also been discovered in several non-hematopoietic tissues and can be used to enrich other progenitor cell populations as well. It is suggested that Sca-1 could be involved in regulating both B and T cell activation.