Pulmonary carcinoids show immunoreactivity for neuroendocrine markers, including synaptophysin (cytoplasmic), chromogranin (cytoplasmic; shown here), NCAM/CD56 (cell membrane staining), neuron-specific enolase, serotonin, and neurofilaments. Pancytokeratin is positive, whereas high molecular weight cytokeratin is generally negative. Pulmonary carcinoids are usually negative for TTF-1 (some peripherally-located pulm. carcinoids may be positive). Based on the active peptides produced by a given tumor, there may be positivity for somatostatin, calcitonin, ACTH, gastrin-releasing peptide/bombesin, GHRH, substance P and VIP. Ki-67 labeling index is lower in carcinoids (< 10-20%) as compared to small cell carcinoma and large cell neuroendocrine carcinoma (> 50%, often as high as 80%) and can be a useful adjunct in small biopsies with crush artifact.