The FSHD-associated DUX4 gene encodes at least two different protein isoforms generated through alternative mRNA splicing (Fig 1B): a non-pathogenic “short” form of unknown function (DUX4-S) expressed at low levels in healthy somatic cells and a longer “full-length” form (DUX4-FL) that is expressed in the male germ line and can be highly cytotoxic when expressed in somatic cells [23, 25–28]. The gene discussed is DUX4; the disease is facioscapulohumeral muscular dystrophy.