ICI-85966 (former tentative brand name Stilbostat), also known as diethylstilbestrol (DES) bis(di(2-chloroethyl)carbamate), is a synthetic, nonsteroidal estrogen and cytostatic antineoplastic agent of the stilbestrol group and a nitrogen mustard ester of diethylstilbestrol (DES) which was developed for the treatment of breast cancer and prostate cancer but was never marketed (possibly due to the toxicity of DES).