Revisiting the consequences of deleting the X inactivation center

Significance Mammalian cells equalize X-linked dosages between the male (XY) and female (XX) sexes by silencing one X chromosome in the female sex. X chromosome inactivation (XCI) requires the X inactivation center (Xic ), which spans several hundred kilobases in the mouse and includes several required noncoding genes. Over three decades, transgenic and deletional analyses have demonstrated that theXic is both necessary and sufficient to induce X chromosome counting, choice, and the initiation of whole-chromosome silencing. However, one recent study reported that deleting the noncodingXic surprisingly had no effect on counting or initiation of XCI. Here, we revisited the question, created independentXic deletion lines, and reaffirmed the essential nature of theXic for counting and X chromosome silencing.