4EN1:
reference: The 1.6 A resolution structure of a FRET-optimized Cerulean fluorescent protein., Watkins JL, Kim H, Markwardt ML, Chen L, Fromme R, Rizzo MA, Wachter RM, Acta Crystallogr D Biol Crystallogr. 2013 May;69(Pt 5):767-73. doi:, 10.1107/S0907444913001546. Epub 2013 Apr 11. PMID: 23633585

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