• Paired intermediate filament proteins with highest specificity for epithelial tissues
• Complex epithelium contains higher MW keratins; simple epithelium for lower MW keratins
• Cam 5.2 (low and intermed, MW keratins) best for poorly differentiated tumors
• Pan keratins (AE1/AE3) best screening Ab
• 34BE12-prostatic basal cells
• Cytokeratin 5/6 - Mesothelial
• Cytokeratin 7 - Lung
• Cytokeratin 20 - Colon Ca, Merkel

   1.  Nonepithelial tumors expressing keratin

        a.  Mesothelioma

        b.  Meningioma

        c.  Synovial sarcoma

        d.  Epithelioid sarcoma

        e.  Chordoma

        f.  Some smooth muscle tumors

        g.  Rare melanomas, esthesioneuroblastomas, desmoplastic round cell tumors

• Activation marker
• Not epithelial specific (stains Reed-Sternberg cells, some mesenchymal tumors

• Detected in normal, reactive and neoplastic tissue
• Positive in GI tract, prostate, endometrium, endocervix, urothelium and "C" cells of thyroid

• Activation marker
• Myeloid marker and most epithelial cells, R-S cells
• Useful in pleural effusions (negative in mesothelial cells)

• Epithelial-specific
• Positivity excludes mesothelioma (most)

• Lung adenocarcinoma, small cell, atypical carcinoids
• Thyroid follicular and papillary ca

• Calretinin
• HBME-1
• Cytokeratin 5/6

• Variably positive in breast carcinoma

   1.  Vimentin

        a.  Present in most mesenchymal tumors, including lymphomas, melanomas.

        b.  Ideal tissue control for immune reactivity.

        c.  May be expressed by some carcinomas (renal, lung).

            1)  Desmin

                a)  Most muscle tumors although may be negative up to 50% of leiomyosarcomas

                b)  Skeletal, cardiac, and smooth muscle are positive

   1.  Muscle-specific actin (MSA)

        a.  HHF-35 positive in skeletal, smooth and cardiac muscle and is most
            sensitive marker to identify muscle differentiation.

        b.  Some lymphomas and myofibroblastic lesions will stain.

   2.  Smooth muscle actin (SMA)

        a.  Not specific to smooth muscle, but useful and very sensitive.

   3.  Myoglobin

        a.  Most specific but not very sensitive.

        b.  Most often positive in rhabdoid differentiation.

   4.  Myo-D1

   1.  Factor VIII

        a.  Very specific though not highly sensitive without trypsinization

        b.  May be negative to poorly differentiated angiosarcomas.

   2.  CD31

        a.  Most sensitive endothelial marker, platelets

        b.  Complete specificity

   3.  Ulex lectin

        a.  Useful, but positive in some carcinomas and epithelioid sarcoma

   4.  CD34

        a.  Best for endothelial cells of Kaposi's sarcoma

        b.  Positive in solitary fibrous tumor and DFSP dermatofibrosarcoma                   
            protuberans, GI stromal tumors, hemangiopericytomas, hematopoietic precursor      
            cells

CK7+ CK20 + Transitional Cell carcinoma

CK7+ CK20 - Non-Small Cell Adenocarcinoma of Lung

CK7 - CK20 + Colorectal Carcinoma

CK7 - CK20 - Hepatocellular Carcinoma