Abstract
The control scheme for the expression of X chromosomes in female mammals can be considered to consist of two parts (Lyon 1972, 1974). The first of these involves the initiation of inac-tivation of one X chromosome (Kratzer and Gartler 1978; Monk 1978; Chapman, West, and Adler 1978). This initial choice may be either random, as it is in eutherian embryonic somatic tissues, or preferential, as occurs in extraembryonic components of some species (Takagi 1978; West, Papaioannou, Frels, and Chapman 1978) and in somatic tissues of marsupial mammals (Cooper, Johnston, Murtagh, Sharman, VandeBerg, and Poole 1975). This report will focus on the second part of the problem of X-chromosome differentiation: how genetic repression of the inactive X is maintained and propagated. It will consider specifically some of the insights into the regulatory mechanisms involved in the maintenance of repression that may be gained from experiments designed to derepress genes on the inactive X.
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