Mutations that cause pituitary dwarfism in the mouse reside in the gene encoding the transcription factor growth hormone factor 1 (GHF1 or pit1). These dwarf mice (dw and dw^J) are deficient in growth hormone (GH) and prolactin (PRL) synthesis and exhibit pituitary hypoplasia, suggesting a stem cell defect. With antisense oligonucleotide technology, a cell culture model of this genetic defect was developed. Specific inhibition of GHF1 synthesis by complementary oligonucleotides led to a marked decrease in GH and PRL expression and to a marked decrease in proliferation of somatotrophic cell lines. These results provide direct evidence that the homeodomain protein GHF1 is required not only for the establishment and maintenance of the differentiated phenotype but for cell proliferation as well.