become bound are accessible prior to induction, with the different locations of GR binding between cell types largely correlating with the altered locations of accessible DNA [ 76 ]. The widespread binding model incorporates long-standing predictions that, given the relatively high concentrations of transcription factors and DNA in cells, the majority of factor molecules not bound at high levels to functional targets should be bound instead at lower densities to any accessible parts of the genome [ 77 , 78 ]. These thermodynamic arguments are supported by various lines of evidence suggesting that the concentration of free, unbound factor molecules in nuclei is indeed much lower than suggested by the number of molecules present [ 79 - 82 ]. Such predictions were originally made for the Lac repressor in Escherichia coli and assumed that genome-wide, low occupancy binding would result from the sequence-independent, electrostatic affinity of transcription factors for DNA (K D approximately 10 -6 M). Given the broad sequence-specific recognition properties of most animal transcription factors, however, it is likely that most accessible genomic regions will contain moderate or high affinity (K D &lt; 10 -8 M) recognition sites for many of these proteins [ 23 , 83 ]. The factors whose in vivo binding we have examined are typically expressed at tens of thousands of molecules per cell [ 1 , 84 ] (BDTNP, unpublished data). Thus, thermodynamically, most of these molecules are likely to significantly occupy accessible moderate or high affinity recognition sequences, rather than being bound