The form and function of every living organism on the earth depends on the complex regulation of gene expression. This is carried out by controlling and interdigitating spatial and temporal patterns of gene activity during the life time of eukaryotic organisms. This is most dramatically apparent during early stages of development, when new types of cells and organs are being formed, often during very short time spans. To achieve this, it is vital that developmentally important genes can be kept in inactive or active states which are stably inherited in the soma. Indeed, it is now wellknown...