Because rhEPO has been shown to promote proliferation of human endothelial cells and cerebral arteries express EPOR, in seeking the underlying mechanism of the in vivo rhEPO-induced tumor growth acceleration, we evaluated the microvessel density and tumor cell proliferation using immunohistochemisty staining of anti-CD31 and anti-PCNA antibodies and found significantly higher microvessel density (P<0.05, Fig. 4A–D) and increased tumor cell proliferation (P<0.05, Fig. 4E and F) in rhEPO treated xenograft tumors than control ones. The gene discussed is PECAM1; the disease is neoplasm.