RdRp denotes the RNA-dependent RNA polymerase (RNA replicase); DdRp, the DNA-dependent RNA polymerase (transcriptase); DdDp, the DNA-dependent DNA polymerase (DNA replicase); RdDp, the RNA-dependent DNA polymerase (reverse transcriptase); dual-Rp, the RNA polymerase that recognizes both RNA and DNA as templates; dual-Dp, the DNA polymerase that recognizes both RNA and DNA as templates. Superscripts indicate whether molecules are RNA or DNA.