Initiation of transcription is the major control point for gene expression. The protein factors regulating transcription, the transcription factors, are modular proteins with distinct domains responsible for different functions including DNA binding, ligand binding, nuclear localization, and protein-protein interactions. Abnormal transcription has been associated with various human diseases from neoplasia to birth defects. These can result from chromosomal translocation or transcription factor genes (e.g. Burkitt's lymphoma) or by inappropriate activation of these factors due to synthesis of fusion genes (e.g. BCR-ABL fusion gene in Philadelphia positive leukaemia). More recently, abnormalities in transcription factor genes have been linked to such human birth defects as Waardenburg syndrome, vesicoureteral reflux and craniosynostosis.