“…Immunohistochemical staining for TTF1, when positive, favors pulmonary over gastrointestinal origin for carcinoid tumors, however not all pulmonary primary carcinoid tumors stain positively with TTF1, and gastrointestinal carcinoid tumors with TTF1 positivity, although weak and focal, have rarely been reported. 25,38 In patient 2, the cytomorphological features of the neoplastic cells, including uniform nuclei, indiscernible nucleoli, eosinophilic cytoplasm, and finely stippled to slightly clumped chromatin, were suggestive of a neuroendocrine neoplasm but not specific for a primary origin. The neoplastic cells were immunoreactive for keratin AE1/AE3, synaptophysin, chromogranin, INSM1, and TTF1 but negative for ISL1 and CDX2.…”