Cyclic Urea (CU) binds at the active site of HIV-1 protease while replacing a structural water molecule. CU is shown in light blue and red, catalytic aspartates in dark blue. CU has been developed as alternately to be an antiviral protease inhibitor by Structure-Based Drug Design (Lam et al., Science 263, 380-384, 1994, drawing by Chong-Hwan Chang using Insight II software).