We present a new result on the existence of Hopf subalgebras in the Hopf algebra of Feynman graphs, which are generated by 1PI Green's functions. This means that the coproduct closes on these Green's functions which allows us for example to rederive Dyson's formula in QED relating the renormalized and unrenormalized proper functions via the renormalization constants. In the case of non-abelian gauge theories, we observe the crucial role played by Slavnov-Taylor identities.