  We derive the capacity region of the Gaussian version of Willems's two-user
MAC with conferencing encoders. This setting differs from the classical MAC in
that, prior to each transmission block, the two transmitters can communicate
with each other over noise-free bit-pipes of given capacities. The derivation
requires a new technique for proving the optimality of Gaussian input
distributions in certain mutual information maximizations under a Markov
constraint. We also consider a Costa-type extension of the Gaussian MAC with
conferencing encoders. In this extension, the channel can be described as a
two-user MAC with Gaussian noise and Gaussian interference where the
interference is known non-causally to the encoders but not to the decoder. We
show that as in Costa's setting the interference sequence can be perfectly
canceled, i.e., that the capacity region without interference can be achieved.
