A region upstream of the mouse adenine phosphoribosyltransferase (aprt) gene has a well characterized methylation pattern for HpaII/MspI sites. When an unmethylated plasmid construct containing this region was transfected into P19 mouse teratocarcinoma stem cells appropriate de novo methylation was observed. However, de novo methylation was significantly reduced when this plasmid was introduced into a differentiated derivative of the P19 stem cell line. Finally, a position effect for de novo methylation was shown by demonstrating methylation of a normally unmethylated HpaII/MspI site when it was placed in this upstream region. This system should prove useful for elucidating DNA signals for de novo methylation and changes in DNA methyltransferase activities that occur during cellular differentiation.