Mouse embryonic stem cells (ESCs) require transcriptional regulation to ensure 
rapid proliferation that allows for self-renewal. However, the molecular 
mechanism by which transcriptional factors regulate this rapid proliferation 
remains largely unknown. Here we present data showing that CIBZ, a BTB domain 
zinc finger transcriptional factor, is a key transcriptional regulator for 
regulation of ESC proliferation. Here we show that deletion or siRNA knockdown 
of CIBZ inhibits ESC proliferation. Cell cycle analysis shows that loss of CIBZ 
delays the progression of ESCs through the G1 to S phase transition. Conversely, 
constitutive ectopic expression of exogenous CIBZ in ESCs promotes proliferation 
and accelerates G1/S transition. These findings suggest that regulation of the 
G1/S transition explains, in part, CIBZ-associated ESC proliferation. Our data 
suggest that CIBZ acts through the post-transcriptionally regulates the 
expression of Nanog, a positive regulator of ESC proliferation and G1/S 
transition, but does not affect Oct3/4 and Sox2 protein expression. Notably, 
constitutive overexpression of Nanog partially rescued the proliferation defect 
caused by CIBZ knockdown, indicating the role of CIBZ in ESC proliferation and 
G1/S transition at least in part depends on the Nanog protein level.