Transcriptional inactivation of one X chromosome in mammalian female somatic 
cells leads to condensation of the inactive X chromosome into the 
heterochromatic sex chromatin, or Barr body. Little is known about the molecular 
composition and structure of the Barr body or the mechanisms leading to its 
formation in female nuclei. Because human sera from patients with autoimmune 
diseases often contain antibodies against a variety of cellular components, we 
reasoned that some autoimmune sera may contain antibodies against proteins 
associated with the Barr body. Therefore, we screened autoimmune sera by 
immunofluorescence of human fibroblasts and identified one serum that 
immunostained a distinct nuclear structure with a size and nuclear localization 
consistent with the Barr body. The number of these structures was consistent 
with the number of Barr bodies expected in diploid female fibroblasts containing 
two to five X chromosomes. Immunostaining with the serum followed by 
fluorescence in situ hybridization with a probe against XIST RNA demonstrated 
that the major fluorescent signal from the autoantibody colocalized with XIST 
RNA. Further analysis of the serum showed that it stains human metaphase 
chromosomes and a nuclear structure consistent with the inactive X in female 
mouse fibroblasts. However, it does not exhibit localization to a Barr body-like 
structure in female mouse embryonic stem cells or in cells from female mouse 
E7.5 embryos. The lack of staining of the inactive X in cells from female E7.5 
embryos suggests the antigen(s) may be involved in X inactivation at a stage 
subsequent to initiation of X inactivation. This demonstration of an 
autoantibody recognizing an antigen(s) associated with the Barr body presents a 
strategy for identifying molecular components of the Barr body and examining the 
molecular basis of X inactivation.