Imprinted X-chromosome inactivation: enlightenment from embryos in vivo

There are two forms of X chromosome inactivation (XCI) in the laboratory mouse, random XCI in the fetus and imprinted paternal XCI limited to the extraembryonic tissues supporting the fetal life in utero. Imprinted XCI has been studied extensively because it takes place 14 first in embryogenesis and it may hold clues to the mechanism of control of XCI in general and to the evolution of random XCI. Classical microscopic and biochemical studies of embryos in vivo provide a basis for interpreting the multifaceted information yielded by various inventive approaches and for planning further experiments. (C) 2003 Published by Elsevier Ltd.