Problem: Epigenetic regulation of gene expression is mediated by protein enzymes that covalently modify histones with methyl-, acetyl- or other chemical groups (writers), protein enzymes that remove such modi?cations (erasers) and proteins that bind speci?cally to such histone modi?cations (readers). How does this process silence genes, what is the role of noncoding RNAs (e.g., Xist RNA), and where are silenced genes in the nucleus? Another type of epigenetic regulation is imprinting (silencing) of genes during sperm or egg formation - discuss the function of this gene copy number control process for a mother, a father and their offspring. Note that imprinting occurs in animal species that use a placenta for nourishing an embryo but not in animals that use an egg for supporting embryo development.