X chromosome inactivation, differentiation, and DNA methylation revisited, with a tribute to Susumu Ohno

X chromosome inactivation and DNA methylation are reviewed, with emphasis on the contributions of Susumu Ohno and the predictions made in my 1975 paper (Riggs, 1975) in which I proposed the maintenance methylase model for somatic inheritance of methylation patterns and suggested that DNA methylation would be involved in mammalian X chromosome inactivation and development. The maintenance methylase model is discussed and updated to consider methylation patterns in cell populations that have occasional, stochastic methylation changes by de novo methylation or demethylation, either active or passive. The way station model for the spread of X inactivation by LINE- 1 elements is also considered, and some recent results from my laboratory are briefly reviewed. Copyright (C) 2002 S. Karger AG, Basel.