Transcription factor CP2 was identified initially to bind the promoter region of the murine a-globin gene and its activity was shown to increase 2 to 3 fold during the induced differentiation of murine erythroleukemia (MEL) cells. To get further insight into the role of CP2 during development and differentiation, steady-state levels of CP2 message were monitored by using reverse transcriptase (RT)-PCR and in situ hybridization assays in the cultured MEL cells and differentiating embryonic stem (ES) cells in vitro, and in fetal and adult mouse tissues. The amount of CP2 messages increased 3 to 5 fold during induced differentiation of MEL cells, suggesting that the increment of CP2 activity during induced differentiation of MEL cells is originated from the increase of transcription initiation. On the other hand, CP2 expression is not restricted to the erythroid lineage cells; CP2 expressed ubiquitously from the undifferentiated ES cells to adult tissue cells. CP2 transcript was observed even in the undifferentiated ES cells and the level of expression increased from day 8 of the differentiating embryoid bodies. RT-PCR assay in the total RNAs prepared from several tissues of the adult mouse also showed ubiquitous expression profile, although the levels of expression were variable among tissues. When non-radioactive in situ hybridization assay was performed to the paraffin-sectioned whole body mouse embryos at days 11.5, 13.5, and 16.5 after fertilization, variable amounts of positive signals were also detected in different tissues.