INVITED EDITORIAL The Timing of Twinning: More Insights from X Inactivation

Since the formulation of the hypothesis, by Mary Lyon(1961), of X-chromosome inactivation in somatic cellsof female mammals, studies of the biology and the con-sequences of this phenomenon have contributed to ourunderstanding of many areas of genetics and develop-ment. Montiero et al. (1998 [in this issue]) have nowextended the application of X-inactivation studies to acareful and quantitative dissection of the timing of MZtwinning, demonstrating that dichorionic MZ (DC-MZ)twinning, unlike monochorionic MZ (MC-MZ) twin-ning, occurs prior to the time of X inactivation in theembryo.Recent discoveries have illuminated many details ofthe X-inactivation story, but the precisemoleculareventsin the initiation and maintenance of X inactivation arenot yet fully known. We now appreciate that expressionlevelsofthegreatmajorityofX-encodedgenesareequal-ized between XY males and XX females by permanentsilencing of one or the other X chromosome in the cellsof female somatic tissues. Normal X inactivation occursin the early female embryo as a stochastic event—thatis, a choice made independently in each cell, with anequal probability of the maternally derived versus pa-ternally derived X chromosome becoming inactive. TheX inactivation of individual embryonic cells is consid-ered most likely to be initiated from the X-inactivationcenter by