AbstractA knowledge of the DNA binding protein content of a cell or tissue type can be an important aid towards a more thorough understanding of the functional role of that tissue. Important processes, such as the development of an adult organism from the single-celled zygote and the maintenance of the developed structure and biochemical characteristics of tissues, are increasingly being demonstrated to be dependent on DNA binding transcription factors. The spectrum of transcription factors present determines what genes may be transcribed in a cell type, including genes that encode further transcription factors. The expression of a single gene may be determined by a number of different transcription factors, and the ratio of their concentrations may be vital, particularly if competing positive and negative factors are involved. Factors other than those that bind the DNA may be involved, such as protein cofactors, metal ions, and ligand molecules, and a further important factor is the chromatin structure of the DNA. The gene must be available for binding of factors and subsequent transcription.