The genetic defects that results in TAL1 oncogene activation are commonly seen in leukemic cells of the patient with T-cell Acute Lymphoblastic Leukemia ( T-ALL ). The ectopic expression of TAL1 oncoprotein perturbs the development of T-cell, hence promotes the formation of leukemia. TAL1 gene encodes proteins with basic helix-loop-helix ( bHLH ) domain, a protein dimerization and DNA binding domain. In T-ALL cells, two Tal1 proteins, pp42(1-331 amino acids) and pp22(176-331 amino acids) are produced that both contain bHLH domain. Both proteins interact with immunoglobulin gene enhancer binding protein, E12/E47 to form DNA-binding heterodimers, that can bind to consensus E-box DNA sequence AACAGATGGT. Phosphorylation of S122 residue modulates the trans-activation potential of Tal1 protein. In addition, S172 is an inducible c-AMP dependent protein kinase (PKA) phosphorylation site in vivo. The phosphorylation of TAL1 S172 upon stimulation by forskolin can increase the DNA binding of E12-Tal1 heterodimer. We used site-directed mutagenesis to investigate the effect of S194,S224 mutation on the function of truncated Tal1 oncoprotein.Mutant Tal1 and E12 proteins expression plasmids were constructed and introduced into COS-1 cells by cotransfection. Tal1 and E12 protein expression in transfected cell were evaluated by Western blotting. The protein-DNA interaction were evaluated by electrophorectic mobility shift assay. The mutation of S194 and S224 of Tal1 protein all resulted in the loss of DNA-binding complex formation. This data indicated that these serine residues are essential for bHLH function. However, the phosphorylation status of these two residues in vivo, and what kinase is responsible for the phosphorylation of these residues, await further investigation.