A Barr body, also known as X-chromatin, is an inactive X chromosome. In species with XY sex-determination, females typically have two X chromosomes, and one is rendered inactive in a process called lyonization. The Barr body can be seen in the interphase nucleus as a darkly staining small mass in contact with the nucleus membrane. It is present in a large proportion of nuclei of female mammals. In humans with euploidy, a genotypical female (46, XX karyotype) has one Barr body per somatic cell nucleus, while a genotypical male (46, XY) has none. In humans with more than one X chromosome, the number of Barr bodies visible at interphase is always one fewer than the total number of X chromosomes. For example, people with Klinefelter syndrome (47, XXY) have a single Barr body, and people with a 47, XXX karyotype have two Barr bodies.

The Barr body is formed by a process called X-inactivation, which occurs early in embryonic development. It is thought that this constitutes the mechanism of choice, and allows downstream processes to establish the compact state of the Barr body. These changes include histone modifications, such as histone H3 methylation and histone H2A ubiquitination, as well as direct modification of the DNA itself, via the methylation of CpG sites. These changes help inactivate gene expression on the inactive X-chromosome and to balance gene expression between males and females.

Barr body testing was used in the 1968 Olympic games in an effort to detect male athletes supposedly trying to "pass" as females to gain a competitive advantage. Such allegations had been made for many years, and a number of athletes were stripped of their medals as a result of ambiguous genital sex. Barr Body testing never detected deliberate fakery. It did, however, detect instances of Androgen Insensitivity Syndrome (AIS), a genetic condition in which an XY zygote develops as a phenotypically female adult, due to failure of androgen receptors. Such individuals would test negative for the presence of a Barr body.