4YDF:
reference: Privileged Structures Meet Human T-Cell Leukemia Virus-1 (HTLV-1): C2-Symmetric 3,4-Disubstituted Pyrrolidines as Nonpeptidic HTLV-1 Protease Inhibitors., Kuhnert M, Blum A, Steuber H, Diederich WE, J Med Chem. 2015 Jun 11;58(11):4845-50. doi: 10.1021/acs.jmedchem.5b00346. Epub, 2015 May 22. PMID: 26000468

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