The mechanism of maternal mitochondrial DNA (mtDNA) inheritance in animals can be said to be the selective elimination of sperm mtDNA via the elimination factor of the egg and a sperm mitochondria-specific factor. In 2005, we clarified that t-tpis (Spagl isoform 1) is a mitochondria-specific translocator and the sperm factor, and furthermore estimated that the elimination factors of the egg are the divalent cation-dependent endonuclease and s-tpis (Spagl isoform 2 and isoform 3) as the elimination system-specific chaperone [K. Hayashida, K. Omagari, J. Masuda, H. Hazama, Y. Kadokawa, K. Ohba, S. Kohno, The sperm mitochondria-specific translocator has a key role in maternal mitochondrial inheritance, Cell Biol. Int. 29 (2005) 472-481]. This time, using a recombinant Spagl isoform I protein, a pull-down assay of ovary cytosol was performed and the elimination factors searched for. Surprisingly, an endogenous retroviral integrase fragment (Eril 5) was identified using mass spectrometry of the electrophoresis band of the pull-down protein. Eril5 was detected as a complex of similar to 500 kDa with Spagl isoform 2 or isoform 3 in native PAGE of the ovary cytosol. This strongly suggested that Eril5 is selectively transported into the sperm mitochondria matrix by Spagl isoform 2 and 3 via Spagl isoform 1 and that sperm mtDNA is destroyed, thus causing the establishment of maternal mtDNA inheritance. (c) 2007 Elsevier Inc. All rights reserved.