Nanog is involved in controlling pluripotency and differentiation of stem cells 
in vitro. However, its function in vivo has been studied only in mouse embryos 
and various reports suggest that Nanog may not be required for the regulation of 
differentiation. To better understand endogenous Nanog function, more animal 
models should be introduced to complement the murine model. Here, we have 
identified the homolog of the mammalian Nanog gene in teleost fish and describe 
the endogenous expression of Ol-Nanog mRNA and protein during medaka (Oryzias 
latipes) embryonic development and in the adult gonads. Using medaka fish as a 
vertebrate model to study Nanog function, we demonstrate that Ol-Nanog is 
necessary for S-phase transition and proliferation in the developing embryo. 
Moreover, inhibition or overexpression of Ol-Nanog does not affect gene 
expression of various pluripotency and differentiation markers, suggesting that 
this transcription factor may not play a direct role in embryonic germ layer 
differentiation. STEM CELLS 2009;27:2081-2091.