Ubiquitination-mediated protein degradation of key transcriptional factors is important to the self-renewal of embryonic stem (ES) cells. However, little is known about the deubiquitination in ES self-renewal and differentiation. Here, we report that deubiquitinase USP21 is an important positive regulator to keep ES cells under undifferentiation stasus by deubiquitination and stabilization of Nanog, a key transcriptional factor of ES cells. Loss of USP21 led to ES cells differentiation and defect in reprogramming. Overall design: Gene expression profiles of mouse embryonic stem cell after USP21 knockdown were generated by deep sequencing, using Illumina HighSeq2000, respectively.