Key stages of genomic imprinting during development. (Erasure) The “imprint” (inherited from the previous generation, generation one) is erased on both parental chromosomes during germ cell development (note that this figure shows complete erasure of all imprints in the germ line, which is one of two models discussed in the text). (Establishment) A new imprint is established (+ or −), according to the sex of the germ line, for the next generation (generation two). (Maintenance) The imprint is stably propagated during mitosis; although not part of the developmental cycle of imprinting, the maternal (m) and paternal (p) imprints are translated into monoallelic expression (indicated by arrow) during development (Reading). Successive cycles of erasure, establishment, and maintenance are a key characteristic of the imprinting process.
