Duchenne Muscular Dystrophy (DMD) and its associated cardiomyopathy primarily affect males. However, 8-10% of female carriers of DMD are symptomatic and under-represented in research due to the lack of a faithful mouse model. To generate a symptomatic mouse model of DMD carriers, we injected mdx embryonic stem cells (ESCs) into wild-type (WT) blastocysts (mdx/WT chimera). In the heart, mdx/WT chimeras develop fibrotic cardiomyopathy. We found that Connexin-43 (Cx43), the primary gap junctional protein in the heart, was pathologically enhanced and mislocalized in mdx/WT chimeras. In the skeletal muscle, we found evidence of fibrosis, inflammation and muscle weakness. Both pathologies in the heart and skeletal muscle are characteristic of symptomatic DMD female carriers. Cx43 was also enhanced in the dystrophic skeletal muscle. Genetic reduction of Cx43-copy number protected mdx/WT chimeras from cardiac and skeletal muscle fiber damage. The latter result was unexpected because Cx43 is not expressed in mature muscle fibers. However, Cx43 expression was detected in the interstitial space between muscle fibers in mononuclear cells invading the dystrophic skeletal muscle. Pathologically enhanced activity of Cx43 in mdx FACS-macrophages was observed via ethidium bromide uptake and the GAP19 Cx43 hemichannel peptide mimetic inhibited Cx43 function in a dose-dependent manner. Because an excess of Cx43 has been associated with cell death, we believe that Cx43 reduction in invading mdx macrophages benefits the skeletal muscle of understudied DMD carriers, perhaps by a mechanism involving macrophage-skeletal muscle fiber communication.