It has been known that aberrant isoforms of Ikaros family ptoteins are expressed in malignant cells. We identified that PBMC of ATL (acute T-cell leukemia) patients expresses different patterns of Helios isoforms compared with normal PBMC. In order to investigate the biological impact of ATL-type Helios isoform, we conducted complehensive gene expression analyses in Jurkat cells overexpressing ATL-type Helios. Also, we investigated the influence of WT-Helios or WT-Ikaros knockdown on the gene expression profile and compared with that of the cells with ATL-type Helios overexpression. Total RNA samples from Jurkat cells were subjected to Cy-3 labeling followed by human whole genome gene expression microarray analyses.