Human acute myeloblastic leukemia HL-60 cells become resistant to differentiation during long­term cultivation. After 150 passages, double minute chromosomes (dmins) found in early­-passaged cells are replaced by large extrachromosomal elements (LEEs). In a DNA library derived from a purified fraction of LEEs, 12.6% (23/183) of clones were assigned to 8q24 and 9.2% (17/183) were assigned to 14q11 in the human genome. Fluorescence in situ hybridization (FISH) revealed a small aberrant chromosome, which had not been found in early­-passaged cells, in addition to the purified LEEs. We determined that each LEE consisted of six discontinuous segments in a region that extended for 4.4 Mb over the 8q24 locus. Five genes, namely, Myc (a proto­-oncogene), NSMCE2 (for a SUMO ligase), CCDC26 (for a retinoic acid­-dependent modulator of myeloid differentiation), TRIB1 (for a regulator of MAPK kinase) and LOC389637 (for a protein of unknown function), were encoded by the amplicon. Breaks in the chromosomal DNA within the amplicon were found in the NSMCE2 and CCDC26 genes. The discontinuous structure of the amplicon unit of the LEEs was identical with that of dmins in HL­60 early­-passaged cells. The difference between them seemed, predominantly, to be the number (10 to 15 copies per LEE versus2 or 3 copies per dmin) of constituent units. Expression of the Myc, NSMCE2, CCDC26 and LOC389637 and TRIB1genes was constitutive in all lines of HL­-60 cells and that of the first four genes was repressed during the terminal differentiation of early­passaged HL­-60 cells. We also detected abnormal transcripts of CCDC26. Our results suggest that these genes were selected during the development of amplicons. They might be amplified and, sometimes, truncated to contribute to the maintenance of HL­-60 cells in an undifferentiated state.