CD34 is a type of glycoprotein found on the surface of cells, specifically hematopoietic progenitor cells, cells that line blood vessels, and fibroblasts in embryos. 1 While the precise role of CD34 is still unclear, research suggests it might boost the growth of stem or progenitor cells while preventing them from maturing, as well as aid lymphocytes in sticking to the lining of blood vessels within lymphoid organs. 2 Expression of CD34 has also been noted in particular gastrointestinal stromal tumors (GIST).