A key issue in understanding the hematopoietic system and B cell biology is to define the function of transcription factors. B lymphocyte development and function is controlled by a hierarchy of transcription factors including PU.1, Ikaros, E2A, EBF, Pax5 and Aiolos. Mouse knockout models provide information about the developmental and physiological importance of the disrupted gene. However, an early block in the development or a lethal phenotype prevents the studies of the functional importance of the gene at the later developing system such as the immune system. The chicken B cell line DT40 is used to circumvent these problems. Studies with DT40 have revealed a role for Ikaros transcription factor in B cell receptor signaling and Aiolos has been shown to regulate immunoglobulin gene conversion and cell survival. On the other hand, findings with Pax5 deficient mutants support DT40 targeting system as a valid model for the plasma cell differentiation and demonstrate the genetic plasticity of the cell line. This system is an excellent model to study transcription factors in B cell specific functions, antibody production and B cell differentiation.