However, the most specific and practically pathognomonic finding for MCC is the characteristic paranuclear staining pattern of cytokeratin 20, which is observed in more than 95% of MCC cases and serves as the most reliable diagnostic differentiator from other neuroendocrine neoplasms, particularly small cell lung carcinoma metastasizing to the skin, which typically shows diffuse cytoplasmic or negative CK20 staining [11,12]. The gene discussed is KRT20; the disease is small cell lung carcinoma.