Emerin induces nuclear breakage in Xenopus extract and early embryos. Emerin is an inner nuclear membrane protein often mutated in Emery-Dreifuss muscular dystrophy. Because emerin has diverse roles in nuclear mechanics, cytoskeletal organization, and gene expression, it has been difficult to elucidate emerin''s contribution to nuclear structure and disease pathology. In this study, we investigated emerin''s impact on nuclei assembled in Xenopus laevis egg extract, a simplified biochemical system that lacks potentially confounding cellular factors and activities. Notably, these extracts are transcriptionally inert and lack endogenous emerin and filamentous actin. Strikingly, emerin caused rupture of egg extract nuclei dependent on the application of shear force. In egg extract, emerin localized to non-nuclear cytoplasmic membranes, and nuclear rupture was rescued by targeting emerin to the nucleus, disrupting its membrane association, or assembling nuclei with lamin A. Furthermore, emerin induced breakage of nuclei in early stage X. laevis embryo extracts, and embryos microinjected with emerin were inviable with ruptured nuclei. We propose that cytoplasmic membrane localization of emerin leads to rupture of nuclei that are more sensitive to mechanical perturbation, findings that may be relevant to early development and certain laminopathies. [Media: see text] [Media: see text] [Media: see text].