Less than 2% of the DNA in human genomes contains conventional genes that code for proteins. Much of the remaining DNA consists of sequences frequently referred to as “junk”. This category includes repeated sequences that we know little about but have links to cancer and other diseases. These repetitive genomic regions can go awry causing catastrophic genomic rearrangements and some can even behave like viruses. In this science cafe, we’ll explore the mysterious parts of genomes rich in so-called “junk” DNA. We’ll discuss the controversy over what is considered a “functional genetic element” and how studying genomic differences within and between species helps shed light on a sequence’s functional significance.