In 1970 Baltimore and Temin discovered that certain viruses that have their genes in the form of RNA can copy the RNA "backward" into DNA in infected cells. The enyzme, reverse transcriptase, enables the manufacture of specific proteins for use as medicines.

Baltimore, who was the only author of this paper, shared the 1975 Nobel Prize in Physiology or Medicine with R. Dulbecco and H. M. Temin "for their discoveries concerning the interaction between tumour viruses and the genetic material of the cell."

The Baltimore classification of virus groups places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication.

• 1.1Group I: Double-stranded DNA viruses
• 1.2Group II: Single-stranded DNA viruses
• 1.3Group III: Double-stranded RNA viruses
• 1.4Group IV & V: Single-stranded RNA viruses
  • 1.4.1Group IV: Single-stranded RNA viruses - Positive-sense
  • 1.4.2Group V: Single-stranded RNA viruses - Negative-sense
• 1.5Group VI: Positive-sense single-stranded RNA viruses that replicate through a DNA intermediate
• 1.6Group VII: Double-stranded DNA viruses that replicate through a single-stranded RNA intermediate

Digital facsimile from PubMedCentral at this link.

The poliovirus genome. Digital facsimile from PNAS through PubMedCentral at this link.