PECAM1 and neoplasm: To test our hypothesis that D2R agonist treatment reduces SCLC tumour growth by reducing tumour angiogenesis and creating a less favourable tumour microenvironment that limits cancer cell proliferation, we co-stained tumour specimens harvested from the mice for CD31 to detect tumour-associated endothelial cells and for TUNEL (terminal deoxynucleotidyl transferase–mediated deoxyuridine triphosphate nick end labelling) to quantify apoptosis.