Stochastic/genetic skewing of X chromosome inactivation. (A) Both X chromosomes are active in early embryonic stem cells. Crosshatching represents the active X chromosomes of maternal (black) and paternal (white) origin. (B) X chromosome inactivation choice is stochastic resulting in (C) variance of the maternal/paternal inactivated X chromosome ratio from the theoretical 1:1. (D) Genetic factors can also affect the choice of X chromosome inactivated, resulting in skewed maternal/paternal inactivated X chromosome ratios. These skewed populations can result in a false determination of clonality.