The orderly and sequential activation of genes during development is hypothesized to be related to the selective expression of groups of regulatory proteins acting primarily at the level of transcription. A nuclear protein was found in hepatocytes, but not other cell types, that binds to a sequence required for hepatocyte-specific transcription of the gene for the β chain of fibrinogen. This protein, hepatocyte nuclear factor 1 (HNF1), also interacts with homologous sequences required for optimal promoter function of the genes for the α chain of fibrinogen and α₁-antitrypsin. The promoter or enhancer regions for several viral and cellular genes not expressed in the liver did not compete for this binding. The restricted expression of HNF1 and its selective interaction with the control regions of several liver-specific genes indicate that it is involved in developmentally regulated gene expression in the liver.