Intracellular mitochondrial DNA transfers to the nucleus in human cancer cells. 
Genome instability is a well-known hallmark of cancer cells.  With the revolution of high-throughput sequencing technologies, our knowledge of somatically acquired genome structural variation (SV) has greatly improved over the last decade.  Remarkably, surveys of thousands of human whole-cancer genomes have shown that chromosomal rearrangements are frequently combined with mitochondrial DNA (mtDNA) fragments somatically transferred to the nucleus.  The high transfer rate and features of integration breakpoints provide clues for understanding the potential mechanisms underlying these events and provide insights into the role of mtDNA segments transferred into the nucleus.  In this review, I discuss our current understanding of somatic nuclear transfer of mitochondrial DNA into the nuclear genome of human cancer cells.