About ~99% of mitochondrial proteins are encoded by nuclear genes, synthesised in the cytosol and are imported into the organelle. Tom40, the core component of the TOM (Translocase of the outer membrane) complex forms a central channel for precursor translocation across the mitochondrial outer membrane and it is essential for cell viability. Given the indispensable role of Tom40 in protein import into mitochondria, mutations in the mammalian Tom40 gene are likely to have pathological consequences. The work outlined in this project mainly focusses on characterizing mitochondria from stilliom (Tom40 mutant) mouse strain which die early from pulmonary vascular congestion. Genomic sequencing revealed a mutation in the TOMM40 gene resulting in a substitution of an alanine for a valine at amino acid 326 (Tom40A326V). The alteration in Tom40 protein causes a change in its electrophoretic mobility on SDS-PAGE and the total Tom40 protein levels in the stilliom mouse tissues were reduced. Biochemical studies demonstrated that the single mutation in Tom40 leads to the destabilization of the TOM complex. Interestingly, 2-dimensional PAGE analysis show that the Tom40A326V can stably associate with small TOM subunits, Tom5, Tom6 and Tom7, however, its interaction with the central organizer subunit, Tom22, was significantly reduced. Taken together, these observations indicate that the A326V mutation affects the stability of the TOM complex. The majority of the mutagenesis studies on Tom40 performed previously were based on fungal models and Stilliom is the first mouse model unveiling a correlation of defects in Tom40 with tissue dysfunction, pointing to new insights into potential disease pathogenesis.