A novel cellular enhancer nucleotide sequence causes expression in undifferentiated stem cells of a flanking exogenous or recombinant gene from a promoter accompanying the gene where the gene and promoter are not normally expressed in the undifferentiated stem cells. In the preferred example the essential or basic core of the cellular enhancer nucleotide sequence is ##STR1## The cellular enhancer may encompass a more general core sequence of approximately, for example, in the range of 300 to 350 bases including the essential or basic core sequence. Recombinant vectors including plasmids and viruses are constructed bearing the novel cellular enhancer flanking a recombinant or exogenous gene and promoter having a specified phenotypic trait to be expressed in undifferentiated stem cells. Propagating cells containing the vector constructs reproduce and propagate the vectors. Undifferentiated stem cells including cell lines and animals express the desired phenotypic trait of an exogenous or recombinant gene under the control of the novel cellular enhancer.