Transposable genetic elements (TEs) comprise a vast array of DNA sequences, all having the ability to move to new sites in genomes either directly by a cut-and-paste mechanism (transposons) or indirectly through an RNA intermediate (retrotransposons). First discovered in maize plants by the brilliant geneticist Barbara McClintock in the mid-1940s, they were initially considered something of a genetic oddity (1, 2). Several decades later, TEs acquired the anthropomorphic labels of “selfish” and “parasitic” because of their replicative autonomy and potential for genetic disruption (3, 4). However, TEs generally exist in eukaryotic genomes in a reversibly inactive, genetically undetectable form we now call “epigenetically silenced,” whose discovery can also be traced to McClintock's elegant genetic studies (5, 6). As the underlying biochemical mechanisms emerged from obscurity and epigenetics became popular toward the end of the 20th century, it was proposed that epigenetic silencing evolved to control the proliferation of TEs and their perceived destructive potential (5, 6).