Viral reverse transcriptase (RT) plays a critical role in replication (e.g., retroviruses, that reverse transcribe RNA templates into complementary DNA) and genome mutations (e.g., diversity-generating retroelements in bacteriophages). In contrast to retroviruses, bacterial viruses (bacteriophages) use RT not for genome replication, but for viral gene variation within diversity-generating retroelements (DGRs) or for other diverse biological functions (e.g., CRISPR-RT).