C4 zinc-finger, basic-zipper, and helix-loop-helix proteins can form heterodimers with other members of the same class. (a) In the example shown, factors A, B, and C can each interact with each other, permitting the three transcription factors to bind to six different DNA sequences (sites 1-6) and creating six combinations of activation domains. (Each binding site is divided into two half-sites). Four different factor monomers could combine to make 12 homo- and heterodimeric factors; five monomers could make 20 dimeric factors; and so forth. (b) When an inhibitory factor (green) is expressed that interacts only with factor A, binding to sites 1, 4, and 5 is inhibited, but binding to sites 2, 3, and 6 is unaffected.