The important consideration in these cases is not the horn, but the underlying pathology which may be benign (Seborrheic keratosis, viral warts, histiocytoma, inverted follicular keratosis
, verrucous epidermal nevus, molluscum contagiosum, etc.), premalignant (Solar keratosis, arsenical keratosis, Bowen's disease) or malignant (Squamous cell carcinoma, rarely basal cell carcinoma, metastatic renal carcinoma, granular cell tumour, sebaceous carcinoma or Kaposi's sarcoma.
(2013) using BerEP4 to distinguish 24 BCC specimens with 88 common skin neoplasms, namely, trichoepithelioma, actinic keratosis, squamous cell carcinoma in situ, squamous cell carcinoma, seborrheic keratosis, lichen planus like keratosis, nevi, hemangioma, inverted follicular keratosis, sebaceous adenoma, and Merkel cell carcinoma (MCC) from dermatopathology files, also showed that it was able to diffusely stain all BCC sections.
Actinic keratosis , seborrheic keratosis ,poroma , lichen planus like keratosis , nevi , hemangioma , inverted follicular keratosis , squamous intraepithelial neoplasia (SIN) , and sebaceous adenoma/hyperplasia  are a wide variety of epithelial skin disorders that have been proven to give negative results on BerEP4 testing (Table 2).
Notably, inherited keratin mutations causing type 2 pachyonychia congenita can produce EVHC along with a characteristic nail dystrophy, focal palmoplantar keratoderma, follicular keratosis
, and multiple steatocysts.
Within this category, diagnoses may include irritated seborrheic keratosis, (6) inverted follicular keratosis, (7) and endophytic pattern verruca vulgaris.
Inverted follicular keratosis is an irritated seborrheic keratosis.
Type 1 - PC1: Jadassohn-Lewandowsky syndrome which is characterized by focal palmoplantar keratoderma (PPK), follicular keratosis
mainly on the trunk and oral leucokeratosis.
Inverted Follicular Keratosis. Inverted follicular keratosis is a lesion that histopathologically shows downgrowths of follicular squamous epithelium and the adjacent epidermis .
Mehregan, "Inverted follicular keratosis is a distinct follicular tumor," The American Journal of Dermatopathology, vol.
Clinical examination revealed follicular keratosis and broken twisted hair (corkscrew hair), in addition to hemorrhagic lesions.
Apart from hemorrhagic lesions, follicular keratosis was noted and hair was twisted and fragmented (Figure 1, marked with arrow).
Age, y/Sex Site Slide 1 11 70/M Nose AK 29 75/M Ear Inverted follicular keratosis
63 27/F Nose Cyst 72 73/F Cheek AK 84 50/F Scalp SK, inflamed 95 19/M Face Lentigo 147 62/F Face AK 203 32/M Trunk Nevus 215 55/F Nose Fibrous papule 279 42/M Forearm Nevus 306 60/M Scrotum Epidermal hyperplasia 310 45/F Thigh Lentigo 395 69/M Shoulder AK 459 79/F Nose AK 464 83/M Ear AK Case No.