Mycosis fungoides (MF) is the most common type of primary cutaneous T-cell lymphomas (CTCL), which are a heterogeneous group of malignancies derived from skin-homing T cells. MF presents in the skin with erythematous patches, plaques, and less frequently, tumours. Although the aetiologies of MF are unknown, important insights have been gained in the immunological and genetic perturbations that are associated with these diseases. Mutations in the p53, p15, p16, JunB, and PTEN genes generally occur in later-stage disease. Loss of normal apoptotic T-cell pathways has also been reported. Apoptosis is partly mediated by death receptors, notably Fas, which is part of the tumor necrosis factor family of receptors. Decreased and/or defective Fas expression by neoplastic T cells has been associated with advanced/aggressive disease and impaired Fas-mediated apoptosis.