Characterization of genomic structures and expression profiles of three tandem repeats of a mouse double homeobox gene: Duxbl. Wu SL et al. We identified and cloned a mouse double homeobox gene (Duxbl), which encodes two homeodomains. Duxbl gene, a tandem triplicate produces two major transcripts, Duxbl and Duxbl-s. The amino acid sequences of Duxbl homeodomains are most similar to those of human DUX4 protein, associated with facioscapulohumeral muscular dystrophy. In adult tissues, Duxbl is predominantly expressed in female reproductive organs and eyes, and slightly expressed in brain and testes. During gonad development, Duxbl is expressed from embryonic to adult stages and specifically expressed in oocytes and spermatogonia. During embryonic development, Duxbl is transcribed in limbs and tail. However, Duxbl proteins were only detected in trunk and limb muscles and in elongated myocytes and myotubes. In C2C12 muscle cell line, Duxbl expression pattern is similar to differentiated marker gene, Myogenin, increased in expression from 2 days onward in differentiating medium. We suggest that Duxbl proteins play regulatory roles during myogenesis and reproductive developments. Developmental Dynamics, 2010. (c) 2010 Wiley-Liss, Inc.