The process of keratin formation in the stratum corneum is called keratinization, which is the transformation of keratin from a cellular protein to a keratin with completely different physicochemical properties. The process of keratinization includes the protofibrillar transformation of the cytoplasm and the breakdown and disappearance of the cytoplasm and nucleus. Etiology The disease can be induced by sunlight, ultraviolet light, radioactive heat, as well as asphalt or coal and their extracts, with the patient’s susceptibility playing a decisive role. Histopathology can be divided into 3 types: hypertrophic atrophic carcinoma-in-situ type. Hypertrophic type: hyperkeratosis of the middle epidermis is obvious, and incomplete keratinization can be seen, with disorderly arrangement of spiny layer cells between hypertrophy and atrophy. Nuclear schizophrenia is more common but atypical with anomalous cells. Epidermal atrophic type: atypical cells and dyskeratotic cells with loose spines were seen in the basal layer. Carcinoma-in-situ type: thickening of the middle epidermis, disorganized epidermal cell arrangement and atypical cells, clear boundary between epidermis and dermis. All three types had obvious elastic degeneration in the superficial dermis, with moderate density infiltration dominated by lymphocytes.