A retroviral protease inhibiting compound of formula (A) wherein R1 is 2-isopropylthiazol-4-yl; n is 1; R2 is hydrogen; R3 is isopropyl; R4 and R4a are phenyl; R6 is hydrogen; R7 is thiazol-5-yl; X is hydrogen and Y is -OH; and Z is -N(CH3)-; or a pharmaceutically acceptable salt, ester or prodrug thereof.