Decrease in DNA methylase activity during preimplantation development in the mouse.

During early mouse development, there are large-scale changes in DNA methylation. These changes may be due to the availability or stability of the enzyme, DNA methyltransferase (methylase), which is responsible for maintenance of DNA methylation. A microassay for methylase activity in preimplantation embryos shows that the level of maternally inherited enzyme is extremely high in the egg and that this activity is stable for the first three cleavage divisions. However, from the 8-cell to the blastocyst stage, there is a marked and absolute decrease in enzyme activity.