X chromosome inactivation (XCI) is the process by which one of the two copies of the X chromosome present in females is inactivated in order to achieves dosage equivalency for X-linked genes between XY males and XX females. An individual cell's decision to inactivate either the paternal or maternal X is made early in development, at approximately the time of implantation, such that normally there is a roughly equal distribution of either X being active. A deviation from equality is known as skewing, with common criteria for "skewed" inactivation being arbitrarily defined as the observation of inactivation of the same allele in 75% or 80% of cells (and very skewed inactivation resulting in 90% or 95% of cells with the same allele inactive). For some X-linked diseases there is strong selection in the heterozygous carrier in favor of cells bearing the mutation on the silenced inactive X, and thus, assessment of XCI skewing in female family members can be suggestive of carrier status.