In higher vertebrates, from amphibians to humans, epidemial maturation is a conserved developmental process. Using adult epidemial tissue and an established keratinocyte cell line, the mouse Nkx-2.3 homeobox gene was demonstrated, for the first time, to be expressed in mouse epidermal keratinocytes. Under the normal culture condition, the spontaneous aggregation phenomenon, a common initiation step of ES cell differentiation, and the induction of mouse adult K1 keratin, a marker of mature epidermal keratinocytes, were both observed in vitro when the Xenopus Nkx-2.3 gene was stably transfected into a mouse pluripotent P19 EC cell line. The induction of mouse K1 keratin by using its Xenopus orthologous gene in the mouse P19 cell implies that Nkx-2.3 may play a conserved role in the epidermal maturation of the mouse, as it does in that of the frog (Ma, 2004). However, the CAT assay study on frog adult keratin promoter could not find the induction of adult keratin. This implies there might not be a direct activation of its promoter.