X chromosome inactivation is reviewed with molecular mechanisms in mind. Models for the various steps leading to the establishment and maintenance of X inactivation are discussed, with comparisons between eutherians and marsupials included. Late DNA replication is proposed to be an epigenetic, self-propagating mechanism aiding the somatic inheritance of determined states, and thus could be the ancestral mechanism for X inactivation. Also, a novel mechanism, Type I DNA-reeling, is proposed to maintain higher order chromosome structure and to help explain the cis-spreading of X inactivation.
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Arthur D. Riggs (1989) studied this question.
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