KRT13 and squamous papilloma: Compared to papillary SCC (Additional file 1: Fig. S1a), benign squamous papilloma consistently shows features favoring non-malignant squamous epithelia, such as intense CK13 expression, predominant 1st parabasal layer Ki-67 detection, and lack of cytoplasmic Ln5γ2-positivity (Additional file 1: Fig. S1b).