['Recent work has demonstrated enhanced X monosomy in women with primary biliary cirrhosis (PBC) as well as two other female-predominant autoimmune diseases, systemic sclerosis and autoimmune thyroid disease. To further our understanding of these events, we have investigated the mechanisms of X chromosome loss and X chromosome inactivation (XCI) in 166 women with PBC and 226 rigorously age-matched healthy and liver disease controls. X chromosome analysis and determination of loss pattern was performed by quantitative fluorescent polymerase chain reaction (QF-PCR) with 4 X-linked short tandem repeats. Further definition of the XCI was based on analysis of methylation-sensitive restriction sites. Importantly, in PBC the X chromosome loss occurs not only more frequently but also in a preferential fashion. This observation supports our thesis that the enhanced X monosomy involves only one parentally derived chromosome and is not secondary to a constitutive non random pattern of XCI. In fact, in the presence of monosomy, the lost X chromosome is necessarily the inactive homologue.'] Do preferential X chromosome loss but random inactivation characterize primary biliary cirrhosis?