Double homeobox 4 (DUX4) is a candidate disease gene for facioscapulohumeral dystrophy (FSHD), one of the most common muscular dystrophies characterized by progressive skeletal muscle degeneration. Despite great strides in understanding precise genetics of FSHD, the molecular pathophysiology of the disease remains unclear. One of the major limitations has been the availability of appropriate molecular tools to study DUX4 protein. In the present study, we report the development of five new monoclonal antibodies targeted against the N- and C-termini of human DUX4, and characterize their reactivity using Western blot and immunofluorescence staining. Additionally, we show that expression of the canonical full coding DUX4 induces cell death in human primary muscle cells, whereas the expression of a shorter splice form of DUX4 results in no such toxicity. Immunostaining with these new antibodies reveals a differential effect of two DUX4 isoforms on human muscle cells. These antibodies will provide an excellent tool for investigating the role of DUX4 in FSHD pathogenesis.