4F2S:
reference: Structural evidence for the rare tautomer hypothesis of spontaneous mutagenesis., Wang W, Hellinga HW, Beese LS, Proc Natl Acad Sci U S A. 2011 Oct 25;108(43):17644-8. Epub 2011 Oct 17. PMID: 22006298

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).