We generalize the Gel'fand-Pinsker model to encompass the setup of a memory less multiple -access channel. According to this setup, only one of the encoders knows the slate of the channel (non-causally), which is also unknown to the receiver. Two independent messages are transmitted: a common message and a message transmitted by the informed encoder. We find explicit characterizations of the capacity region with both non-causal and causal stale information. Further, wc apply the general formula to the Gaussian case with non-causal channel state information, under an individual power constraint as well as a sum power constraint. In this case, the capacity region is achievable by a generalized writing-on-dirty-paper scheme.