4R6L:
reference: Light Signaling Mechanism of Two Tandem Bacteriophytochromes., Yang X, Stojkovic EA, Ozarowski WB, Kuk J, Davydova E, Moffat K, Structure. 2015 Jul 7;23(7):1179-89. doi: 10.1016/j.str.2015.04.022. Epub 2015, Jun 18. PMID: 26095026

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