Paget’s disease: pale cells with adenocarcinoma staining features. Carcinoembryonic antigen (CEA), EMA, CK-7, CK-8, usually positive. Mucin in cytoplasm is often positive with mucicarmine, Alcian blue, colloidal iron, and PAS with and without diastase. Basal cells are often compressed and uninvolved. No dyskeratosis Bowen’s disease (squamous cell carcinoma in situ, 18.10): pale keratinocytes may be present which contain glycogen (PAS positive, diastase labile) with frequent dyskeratosis (1.27). Full-thickness atypia often involves basal cells also. High molecular weight keratin positive. Mucin stains, CEA, EMA, CK-7, CK-8 and low molecular weight keratin negative. However, cases have been published with exceptions, such as positive staining for CK-7 and EMA. Usually no pagetoid cells in the stratum corneum, which sometimes occurs with Paget’s disease and melanoma Borst–Jadassohn phenomenon: discrete clones of basaloid, squamatized, or pale keratinocytes in the epidermis that appear different than their neighbors. This can be benign or malignant. It is mainly seen with irritated seborrheic keratosis (18.2) or Bowen’s disease (18.10), and rarely with hidroacanthoma simplex (a form of eccrine poroma limited to the epidermis, 23.10). Melanoma (20.11): S-100 (very sensitive, but not specific), HMB-45 and MART-1 (both very specific, but not sensitive) stains positive. Fontana melanin stain is also positive, but is less useful because keratinocytes may contain melanin transferred from melanocytes. Melanosomes by electron microscopy Mycosis fungoides (24.1): malignant T-lymphocytes (sometimes cerebriform) in spaces called Pautrier microabscesses. Stains such as CD45 (leukocyte common antigen, used for screening), CD4 (T-helper cells), and CD3 (pan-T cells) usually positive Lymphomatoid papulosis (24.5): CD30 positive large atypical lymphocytes, and most smaller ones in the epidermis stain with T-cell markers Langerhans cell histiocytosis (24.18): malignant Langerhans cells, often kidney-shaped nuclei, CDla or S-100 positive