Mutagenesis screening in mice is an effective means of identifying essential genes in cardiovascular development. The l11Jus8 (L8) mutant mouse line was originally isolated from a region-specific N-ethyl-N-nitrosourea (ENU) chemical mutagenesis screen and exhibited an autosomal recessive mid-gestational embryonic lethal phenotype characterised by haemorrhage in the thoracic cavity, blood pooling in the heart, right ventricular dysmorphology and yolk sac vascular degeneration. Prior work mapped the L8 mutation to a ~2.77Mb region on mouse chromosome 11. The aim of this study was to further characterise the L8 mutant phenotype and identify the L8 causative mutation.Phenotypic characterisation conducted here confirmed mid-gestational lethality, haemorrhage and yolk sac vascular degeneration in L8 mutants. Histological analysis of L8 mutants demonstrated presence of fragmented cell nuclei and loss of myocardial integrity in embryonic atrial myocardium. Areas of fragmented cell nuclei did not exhibit positive staining for apoptosis. Furthermore, L8 mutants did not appear to experience typical cardiac defects in aspects including myocardial or smooth muscle differentiation, cell proliferation, ECM production, myocardial hypoplasia/hyperplasia, basement membrane components or observable aberrations in cardiac conduction. L8 mutants exhibited atypical cardiac defects including sudden cessation of heartbeat with morphological indicators of necrosis such as swelling of mitochondria and release of microparticles both from atrial myocardial cells. The L8 mutant appears to represent a novel combination of cardiac defects or novel defects with secondary cardiac phenotypes.Sequencing of the coding exons and splice junctions of 22 candidate genes within the ~2.77Mb L8 locus did not identify the causative mutation. The L8 locus was therefore further refined to a ~1.16Mb region including 20 genes. Sanger sequencing of 10 of these genes plus targeted sequence capture and SOLiD sequencing of the region did not identify a potential L8 mutation. Given the refinement of the candidate locus and advances in sequencing technology and analysis, further sequencing will likely identify the L8 mutation and confirm the cause of the embryonic lethal phenotype.