DUX4 and Facioscapulohumeral dystrophy: However, it is also important to note that because DUX4 expression has been reported in the testis, thymus, and possibly the skin [5, 6, 45], even a targeted therapeutic approach which envisions utilizing a small molecule to inhibit DUX4 in FSHD muscle may also affect the normal function of some tissues, and potential side effects should be investigated.