Immunostaining further validated the presence of small cell carcinoma, with stains showing the tumor to be positive for pan-keratin, cytokeratin 20 (CK20), synaptophysin, focally positive for chromogranin and thyroid transcription factor 1 (TTF-1), and negative for cytokeratin 7 (CK7) and GATA-3 (Figures 6-8). This evidence concerns the gene KRT20 and small cell carcinoma.