The Ames dwarf mouse transmits a recessive mutation (df) resulting in a profound anterior pituitary hypocellularity due to a general lack of thyrotropes, somatotropes and lactotropes. These cell types are also dependent on the pituitary-specific transcription factor, Pit-1. We present evidence that expression of Pit-1 and limited commitment to these cells lineages occurs in df/df pituitaries. Thus, the crucial role of df may be in lineage-specific proliferation, rather than cytodifferentiation. The presence of all three Pit-1-dependent cell types in clonally derived clusters provides compelling evidence that these three lineages share a common, pluripotent precursor cell. Clusters containing different combinations of Pit-1-dependent cell types suggests that the Pit-1+ precursor cells choose from multiple developmental options during ontogeny. Characterization of df/df<-->+/+ chimeric mice demonstrated that df functions by a cell-autonomous mechanism. Therefore, df and Pit-1 are both cell-autonomous factors required for thyrotrope, somatotrope and lactotrope ontogeny, but their relative roles are different.