Lee elaborates on the early steps of X inactivation. Very early in development, cells “count” the number of X chromosomes and decide if one needs to be inactivated, and if so which one. There is a region of the X chromosome called the X inactivation center which is enriched in long non-coding RNAs (lncRNAs). Lee explains how she and others showed that by sensing the ratio of two specific lncRNAs the cell can determine how many X chromosomes are present. Further studies showed that two different lncRNAs are responsible for randomly determining which X chromosome will be inactivated. Finally, she discusses the hypothesis that the allelic choice mechanism depends on a transient chromosomal pairing event that occurs at the beginning of the dosage compensation process.