Alterations of neuronal apoptosis inhibitory protein (NAIP), a member of the inhibitory of apoptosis protein (IAP) family of inhibitors of apoptosis, have been previously associated with different neurodegenerative disorders. This study indicated the existence of a novel NAIP splice variant. This isoform, NAIP-deltaEx10-11, was found in tumor cell lines of different origin and in normal adult brain. Analysis of the putative protein predicted that the NAIP variant lacks part of the third BIR domain as well as the COOH-terminal tail of regular NAIP. This might suggest that it is endowed with a reduced antiapoptotic activity. This view is supported by the fact that NAIP-deltaEx10-11 mRNA and protein were much less expressed in the multidrug- and apoptosis-resistant HL60R leukemia than in its parental counterpart HL60. The opposite occurred for regular NAIP. Overall, the NAIP transcripts might be involved in tumor resistance to chemotherapeutic agents.