To identify the tissue origin of the carcinoma, we use cytokeratin 7 (CK7), cytokeratin 20 (CK20), thyroid transcription factor 1 (TTF-1), estrogen receptor (ER) in female patients or prostate-specific antigen (PSA) in male patients as the minimal set of markers (Figure 2). The gene discussed is TTF1; the disease is carcinoma.