  Living cells sense their environment through the binding of extra-cellular
molecular ligands to cell surface receptors. Puzzlingly, vast numbers of
signaling pathways exhibit a high degree of cross talk between different
signals whereby different ligands act through the same receptor or shared
components downstream. It remains unclear how a cell can accurately process
information from the environment in such cross-wired pathways. We show that a
feature which commonly accompanies cross talk - signaling pleiotropy (the
ability of a receptor to produce multiple outputs) - offers a solution to the
cross talk problem. In a minimal model we show that a single pleiotropic
receptor can simultaneously identify and accurately sense the concentrations of
arbitrary unknown ligands present individually or in a mixture. We calculate
the fundamental limits of the signaling specificity and accuracy of such
signaling schemes. The model serves as an elementary "building block" towards
understanding more complex cross-wired receptor-ligand signaling networks.
