The envelope glycoproteins of HIV are required for viral infectivity. Proteolysis of the precursor envelope glycoprotein gp160 results in the formation of gp120 and gp41. Cleavage occurs after the sequence Arg-Glu-Lys-Arg. This sequence is expected to be a substrate for the cellular protease furin. We examined whether furin is responsible for cleavage of gp160 by using a furin-deficient CHO cell line and the same cell line transfected with furin cDNA. Data obtained from viral transmission assays suggested that furin increased viral infectivity but was not essential for the maturation of gp160, implying that other proprotein processing enzymes also recognize this putative furin cleavage site.