Two cases of HS occurring in concomitance with Darier’ disease have been recently reported; the authors suggested that a potential causal link between these two conditions may arise from an interaction between Notch homolog 1 and SERCA2, a calcium ATPase encoded by ATP2A2 (ATPase Sarcoplasmic/Endoplasmic Reticulum Ca2+), the gene mutated in Darier’ disease, also known as keratosis follicularis, an autosomal dominant skin disorder characterized by loss of adhesion between epidermal cells and abnormal keratinization [20]. Here, ATP2A2 is linked to histiocytic sarcoma.