KRT20 and Merkel cell skin cancer: MCC cells are CAM 5.2 and cytokeratin (CK) AE1/AE3 positive with a paranuclear, cytoplasmic, or mixed pattern; chromogranin, synaptophysin, CD56, neuron-specific enolase (NSE) positive, but the pathognomonic clue is CK20 positivity with a classic dot-like paranuclear pattern.