DUX4 and facioscapulohumeral muscular dystrophy: Conversely, the D4Z4-2.5 transgenic mouse model, in which a pathogenic FSHD1-sized D4Z4 repeat array consisting of 2.5 D4Z4 repeat units [28, 29], including the distal DUX4 polyadenylation site rendering the distal repeat unit capable of producing the stable DUX4-fl mRNA [4], suffered from too little DUX4 expression in muscle and failed to show an FSHD-like phenotype [23].