text: The discovery of msDNA has led to broader questions regarding where reverse transcriptase originated, as genes encoding for reverse transcriptase (not necessarily associated with msDNA) have been found in prokaryotes, eukaryotes, viruses and even archaea. After a DNA fragment coding for the production of msDNA in E. coli was discovered, it was conjectured that bacteriophages might have been responsible for the introduction of the RT gene into E. coli. These discoveries suggest that reverse transcriptase played a role in the evolution of viruses from bacteria, with one hypothesis stating that, with the help of reverse transcriptase, viruses may have arisen as a breakaway msDNA gene that acquired a protein coat. Since nearly all RT genes function in retrovirus replication and/or the movement of transposable elements, it is reasonable to imagine that retrons might be mobile genetic elements, but there has been little supporting evidence for such a hypothesis, save for the observed fact that msDNA is widely yet sporadically dispersed among bacterial species in a manner suggestive of both horizontal and vertical transfer. Since it is not known whether retron sequences per se represent mobile elements, retrons are functionally defined by their ability to produce msDNA while deliberately avoiding speculation about other possible activities.