(a) A TF preferentially binds to its cognate binding site, but can also bind noncognate sites, potentially causing crosstalk--an erroneous activation or repression of a gene. (b) In a global setting where many TFs regulate many genes, the number of possible noncognate interactions grows quickly with the number of TFs; additionally, it may become difficult to keep TF recognition sequences sufficiently distinct from each other. (c) Cells respond to changing environments by attempting to activate subsets of their genes. In this example, the total number of genes is M = 4, and different environments (here, 6 in total) call for activation of differ- ent subsets with Q = 2 genes. To control the expression in every environment, TFs for Q required genes are present, while the TFs for the remaining M ? Q genes are absent. Because of crosstalk, TFs can bind noncognate sites, generating a pattern of gene expression that can differ from the one required.