Somatic cell DNA methylation patterns in mammals are established during embryonic development and are then maintained somewhat faithfully for the remainder of the individual's lifetime. Pattern formation can be divided into a series of linked steps that include demethylation, de novo methylation, methylation spreading, methylation blocking, and maintenance methylation. In this review, these steps will be combined to present a model for the formation and maintenance of a methylation pattern in the 5' region of the mouse Aprt gene. This model suggests that an apparently 'stable' methylation pattern results from a dynamic equilibrium between forces that promote and inhibit methylation spreading.
Copyright 1999 Academic Press.