CD90 (CDw90, Thy-1) is the 25-35kD smallest member of the immunoglobulin gene superfamily. Human Thy-1 is highly glycosylated and is inserted into the cellular membrane via a glycosyl phosphatidylinositol anchor. The Thy-1 antigen has a broad tissue distribution. It is expressed in nervous tissue, connective tissue and various stomal cell lines. Thy-1 characterizes a rare (0.05 to 0.1%) subset of human fetal bone marrow cells, that contains multipotent hematopoietic progenitor activity. This antigen is expressed on 5 to 25% of CD34+ cells from human bone marrow, umbilical cord blood or fetal liver. CD34+ Thy-1+ cells include the most primitive hematopoietic progenitor cells measured in vitro including high proliferative potential colony-forming cells and cells capable of initiating long-term bone marrow culture.