Gain or loss of genes plays important roles in leukemogenesis of APL via cooperation with PML-RARA. Fluorescence in situ hybridization (FISH) was applied to investigate the DNA copy number changes of hTERT, ERG, CDKN1B (P27), CDKN2A (P16), and TP53 genes in an acute promyelocytic leukemia (APL) cell line (NB4). Five bacterial artificial chromosome probes (BAC) for 9p21.3, 17p13.1, 12p13.2, 5p15.33, 21q22.2 regions were prepared using sequence independent amplification (SIA) and were hybridized to NB4 cells treated with different doses of arsenic trioxide (As(2)O(3); ATO) at various time intervals. NB4 cells were also karyotyped by G-banded chromosome analysis 24 h after culture initiation. FISH analysis prior to treatment showed CDKN1B, CDKN2A, and TP53 gene deletion but ERG and hTERT gene amplification. After treatment with ATO, the number of the NB4 cells with deleted CDKN1B and CDKN2A as well as the counts of the cells with hTERT amplification was significantly reduced in time- and does-dependent manners. In addition, we observed expressive increase in signal patterns of CDKN1B and CDKN2A along with significant decline in hTERT signal patterns in ATO-treated cells as compared with the control group (in time- and dose-dependent manners). On the other hand, no difference in signal patterns for Erg and p53 was observed in response to ATO exposure. The results of the present study show the cytogenetic alteration in hTERT, CDKN1B, and CDKN2A in NB4 cells after treatment with ATO might introduce a new mechanism of antitumor activities of ATO in APL cell line, NB4.