Mammalian cellular differentiation involves a progressive restriction of the developmental potential of immature, multipotent precursor cells into single cell lineages. For example, during haematopoiesis, pluripotent stem cells differentiate into multi potent progenitor cells that ultimately give rise to approximately eight distinct blood cell types. It is clear that lineage-specific transcription factors play an important role both in tissue-specific gene expression and differentiation. The systematic identification of these factors has relied upon the discovery