RPE.40 is a strain of mutated CHO-K1 cells with elevated resistance to Pseudomonas exotoxin A, Sindbis virus, and Newcastle disease virus. Virus resistance is due to an inability to cleave precursor viral membrane glycoproteins and produce infectious virions. Transfection of RPE.40 cells with cDNA for mouse furin causes them to lose all resistance and become as sensitive as wild-type cells to the toxin and viruses. Transfection of RPE.40 cells with cDNA for the related yeast protease Kex2 reduces their resistance to the toxin and viruses, but does not completely eliminate it.