CD90, also known as Thy-1, is a glycosylphosphatidylinositol （GPI）-anchored glycoprotein and a member of the immunoglobulin superfamily with a molecular weight of 25–35 kDa. It is primarily expressed in leukocytes, fibroblasts, and various stromal cells. CD90 plays a crucial role in cell–cell and cell–matrix interactions, and it is involved in processes such as integrin-mediated signaling, cell adhesion, and cellular extravasation. In the context of hematopoietic stem cells, CD90 is often used as a marker for identifying primitive hematopoietic progenitor cells, particularly in combination with other markers like CD34. Additionally, CD90 is expressed on human mesenchymal stem cells （MSCs） from bone marrow and other sources, highlighting its significance in stem cell biology.