Du et al reported that 12 of 14 TTF-1-positive pulmonary carcinoids had a peripheral location with spindle cell morphology, whereas TTF-1-negative carcinoids had a central location.24 This suggests that carcinoids may be classified as distinct subtypes of a central or peripheral origin, like SCLC, and that TTF-1 may be a marker for peripheral carcinoids. This evidence concerns the gene TTF1 and small cell lung carcinoma.