Myeloid cell leukemia-1 (MCL-1) that belongs to BCL-2 family is essential for Survival of hematopoietic stein cells It is upregulated in various types of cancer and promotes cancer cell metastasis. It is known chat human MCL-1 gene undergoes differential splicing and yields three mRNAs encoding antiapoptotic MCL-1L and proapoptotic MCL-1S and MCL-1ES However, no MCL-1 variants have been reported in mousecells We report here anew splicing variant of mouse Mcl-1, Mc1-1V, that is expressed in a variety of mouse normal and tumor cell lines and tissues. Comparative sequence analysis of the full-length Mcl-1 and Mcl-1V cDNAs suggested that Mcl-1V mRNA results from splicing within the first coding exon of Mcl-1 gene at a non-canonical donor-acceptor pair. MCL-1V lacks 46 amino acid residues within the N-terminal region of MCL-1. It localizes in mitochondria and inhibits anoxia- and anticancer drug-induced apoptosis as potent as MCL-1, and decayed less rapidly than MCL-1 in the cells undergoing apoptosis. Collectively, our results show that mouse cells ubiquitously express antiapoptotic MCL-1V that may play a role in mitochondrial cell death. (C) 2009 Elsevier Inc All rights reserved.