 In contrast to calcineurin inhibitors, sirolimus is known to exert remarkable tolerance-promoting properties in multiple animal transplant models. Whether sirolimus is capable of enhancing tolerance-related pathways andor promoting complete withdrawal of immunosuppressive drugs in human transplant recipients has not been previously addressed. The goal of the investigators study is to evaluate the effects of sirolimus on previously identified tolerogenic pathways in humans and, indirectly, to assess the capacity of this drug to enhance the proportion of liver recipients undergoing successful immunosuppression weaning. 