The TCF3/PBX1 fusion oncogene (two transactivation domains of TCF3 fused to the C-terminal portion of PBX1) is under the control of lymphoid-specific promoter/enhancer elements in the pLIT3 expression vector, that include sequences from the proximal promoter of the Lck gene, immunoglobulin enhancer elements, and a TCR Vbeta promoter. The human growth hormone gene with a frameshift mutation in the coding region was inserted 3' of the transgene cDNA to provide introns that enhance the expression. Western blot analysis confirmed expression of the fusion protein in the bone marrow, spleen, and thymus.