Abstract: Using in situ hybridization technique with dig labeled probe,the expression of p21 gene in mice's embryo development has been studied in this paper.The results can be summarized as follows:p21 gene does not take part in the formation of organ primordial during the development of embryo of age 9ds.It starts to express in the embryo of age 10ds and then express stronger with the development of the embryo.It seems to suggest that p21 gene is concord with the negative regulation of cell cycle.The little change in expression strengthen may relates with its stable mRNA.

Key words: in situ hybridization, p21, cell cycle regulation, gene expression