This neoplasm has been shown to exhibit outer root sheath differentiation and is considered to be a malignant form of trichilemmoma. A proliferation of lobular cells continuous with the epidermis, composed of large atypical cells with clear cytoplasm (Figs. 12.2 and 12.3) and PAS positivity, prominent nucleoli, frequent mitoses, and foci of trichilemmal keratinization, is the common pattern (Fig. 12.4). Rhodamine B stain highlights pilar keratin (Fig. 12.5). At the periphery of the lobules, the keratinocytes show palisading and are surrounded by a prominent connective tissue sheath. The tumor is purely intraepithelial, sometimes showing pagetoid spread or is more commonly associated with an invasive component centered around the pilosebaceous unit. Anecdotal cases of TC arising in seborrheic keratosis have been reported. The tumor cells are strongly positive for p53 and focally for CD 34 that is a marker of differentiation from the outer hair sheath. Moreover, TC expresses CK1, CK10, CK14, and CK17, suggesting that it differentiates toward follicular infundibulum. Neuroendocrine positivity with expression of chromogranin, synaptophysin, and CD56 is exceptional.