A ring to bind eukaryotic DNA polymerase Peter H.von Hippel  Structure  Volume 3, Issue 1, Pages 123-124 (January 1995) DOI: 10.1016/S0969-2126(01)00140-X

Figure 1 Comparison of the ribbon structures of (a) a trimer of the yeast PCNA processivity ring with (b) an equivalent view of the ribbon structure of a dimer of the E. coli β ring (both viewed from above with a DNA molecule modeled into the center). Strands of β–sheet are shown as flat ribbon and α–helices as spirals. Individual monomers within each ring are distinguished by different colors. (Reproduced with permission from [1,6] .) . Structure 1995 3, 123-124DOI: (10.1016/S0969-2126(01)00140-X)