For example, ATP induces the reduction of CDP and UDP, dTTP in duces the reduction of GDP and inhibits the reduction of CDP and UDP; and dATP inhibits the reduction of all NDPs. Barry Cooperman has shown that the catalytic activity of mouse ribonucleotide reductase varies with its state of oligomerization, which in turn is governed by the binding of nucleotide effectors to three independent allosteric sites on R1 (Fig. 23-9a): (1) the specificity site, which binds ATP, dATP, dGTP and dTTP; (2) the activity site, which binds ATP and dATP; and (3) the hexamerization site, which binds only ATP. Cooperman's model for the allosteric regulation of Class I ribonucleotide reductase, which quantita- tively accounts for the enzyme's regulatory properties, has the following features (Fig. 23-13)