The availability of pluripotent stem cells offers the possibility of using such cells to model hepatic disease and development. With this in mind, we previously established a protocol that facilitates the differentiation of both human embryonic stem cells and induced pluritpotent cells into cells with hepatocyte characteristics. The use of highly defined culture conditions and the avoidance of feeder cells or embryoid bodies allowed synchronous and reproducible differentiation to occur. The differentiation toward a hepatocytelike fate appeared to recapitulate many of the stages normally associated with the formation of hepatocytes in vivo. In the current study we addressed the feasibility of using human pluripotent stem cells to probe the molecular mechanisms underlying human hepatocyte differentiation. We demonstrate i) that human ES cells express a number of mRNAs that characterize each stage in the differentiation process, ii) that gene expression can be efficiently depleted throughout the differentiation time course using shRNAs expressed from lentiviruses, and iii) that the nuclear hormone receptor HNF4a is essential for specification of human hepatic progenitor cells by establishing expression of the network of transcription factors that control hepatocyte cell fate.