We have previously isolated by expression/selection cloning plasmids containing human cDNAs that rendered MCF-7 breast cancer cells resistant to immunotoxins, Pseudomonas exotoxin (PE), and diphtheria toxin (DT) [Brinkmann et al. (1995) Mol. Med. 1, 206−216]. Here we describe that one of these resistant plasmids, which contains an antisense cDNA fragment homologous to the yeast chromosome segregation gene CSE1 [CAS; Brinkmann et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 10427−10431], reduces the intracellular content of the human CSE1 homologue CAS protein. CAS reduction confers resistance not only to the ADP-ribosylating toxins PE and DT, but also to tumor necrosis factor α and β. The resistance was observed as reduced apoptosis. CAS antisense did not affect the cell death induced by staurosporine, cycloheximide, or etoposide. The observation that CAS antisense can interfere with apoptosis mediated by TNF and ADP-ribosylating toxins suggests that CAS may play a role in selected pathways of apoptosis.