The unique gene expression that defines the hepatocyte conforms to a set of general regulatory principles. The genome encodes the programs of mature gene expression and of the preceding developmental stages. Transcription factors execute these programs by binding to specific DNA sequence motifs grouped together as promoters and enhancers. Expression of each gene therefore reflects the synergistic integration of separate regulations via individual factors. Each cell type expresses a distinctive mixture of hundreds of transcription factors. This mixture, superimposed on the epigenetic history of the cell, determines its phenotype.