CD34 is a transmembrane glycoprotein, which is expressed on immature haematopoietic stem/progenitor cells, capillary endothelial cells and embryonic fibroblasts. It can also be found in splenic marginal zones, dendritic interstitial cells around vessels, nerves, hair follicles, muscle cells and sweat glands in various tissues. CD34 labels capillaries in most tissues but may be absent in large veins and arteries, and is negative in the sinus endothelium of placenta and spleen. CD34 is an excellent indicator of vascular differentiation, regardless of the tumour grade. Prior to the discovery of CD117, CD34 positivity was the best available indicator for a GIST diagnosis, however, it is not a very specific marker for this use. Overall, about 60-70% of GISTs are positive for CD34, although this varies by tumour location: CD34 expression is highest in gastric GISTs (85%), but is only seen in about 50% of small intestinal GISTs (Hasegawa et al; Rudolph). Of the less common locations, CD34 is expressed in 100% of GISTs of the oesophagus, 65% colonic, 96 of rectal GISTs, and 65% of GISTs in non-GI locations, such as the mesentery and omentum (Miettinen et al). In summary, although GISTs often stain for CD34, this staining is not exclusive to GISTs, for example, competing diagnoses, such as schwannoma and solitary fibrous tumour, may also stain for CD34.