Described herein is the finding that increasing the frequency of Zscan4 activation in mouse ES cells not only enhances, but also maintains their developmental potency in long-term cell culture. Particularly disclosed herein is the finding that the constitutive presence of Zscan4-ERT2, even in the absence of its usual activator tamoxifen, can increase the frequency of endogenous Zscan4 activation in ES cells, resulting in the increase of developmental potency of the ES cells. Accordingly, provided herein are Zscan4-ERT2 fusion proteins and nucleic acid molecules and vectors encoding Zscan4-ERT2 fusion proteins. Further provided are methods of prolonging and/or enhancing stem cell plmipotency using the disclosed Zscan4-ERT2 nucleic acid molecules and fusion proteins.