In several cell lines derived from mouse NS-1 myeloma cells, internucleosomal fragmentation of chromosomal DNA, a hallmark of apoptosis, was continuously observed. Approximately 15-20% of the cells died when isolated in a 96-well plate, and the surviving cells contained fragmented DNA ('ladder'). Among a variety of genes so far reported to be related to apoptosis, only Pim-1 was expressed at an elevated level in the NS-1 hybridomas as compared in a control myeloma cell line without 'ladder'. Transfection of a Pim-1 expression vector to a 'ladder'-non-producing myeloma line yielded similar internucleosomal DNA fragmentation. The results hence suggested that Pim-1 activates endonucleases responsible for DNA fragmentation during apoptosis and/or repress DNA repair systems to restore fragmented DNA.