English: Three dimensional structures of the polymerases of the four main RNA viruses groups. In the center of the figure the palm subdomain present in all of the polymerases is shown. (A) single-stranded positive RNA virus: poliovirus RNA-dependent RNA polymerase (PDB code: 3OL6); (B) single-stranded negative RNA virus: influenza A virus RNA-dependent RNA polymerase (PDB code: 4WSB); (C) double-stranded RNA virus: bacteriophage Φ6 RNA-dependent RNA polymerase (PDB code: 1HHS); (D) reverse-transcribing virus: HIV-1 reverse transcriptase (PDB code: 3DLK). (E) The highly conserved palm subdomain showing the position of the conserved catalytic aspartic acid residues (edited from 3DLK). The color code for the subdomains is fingers subdomain, yellow; palm subdomain, green; thumb subdomain, red; fingertips, orange. The conserved structural motifs in the palm subdomain are colored as follows: motif A, red; motif B, dark blue; motif C, green; motif D, magenta; motif E, cyan.