Several general principles have emerged from the study of human transcription factors. First, germline mutations in genes encoding transcription factors result in malformation syndromes in which the development of multiple body structures is affected. Second, somatic mutations involving many of the same genes contribute to tumorigenesis. Third, transcriptional regulatory mechanisms demonstrate remarkable evolutionary conservation. Fourth, prenatal development and postnatal physiology are unified by the demonstration that a single transription factor can control the proliferation of progenitor...