X-Ray structure of human PCNA. Its three subunits, which form a threefold symmetric ring, are drawn in ribbon form. One of these subunits is colored in rainbow order from its N-terminus (blue) to its C-terminus (red), another is pink, and the third is dark green. Click the "Basic Side Chains" button to see the amino acid side chains (shown with C gray, H white, and N blue) that extend toward the center of the trimer and thereby provide the interior surface of the ring with a largely uniform positive charge. Click the "DNA Model" button to show a space-filling model of B-DNA (with C gray, H white, N blue, O red, and P orange) drawn with its helix axis coincident with the PCNA's 3-fold axis. [Based on an X-structure by John Kuriyan, University of California at Berkeley. PDBid 1AXC.]