A Barr body (named after discoverer Murray Barr) is the inactive X chromosome in a female somatic cell, also called as sex chromatin. Human females have two X chromosomes, while males have one X and one Y.

In all of the female somatic cells, one of the X chromosomes is active, and the other is inactivated in a process called lyonization, becoming the Barr body. The reason for shutting off one X chromosome is so that only the necessary amount of genetic information is expressed, rather than double or even more.

Barr body can be observed in female cheek cells by staining with Methylene blue. The nucleus absorbs the stain and appears blue and this Barr body appears in thick blue colour on the edge of the nucleus. In the images taken by foldscope Barr body is very clear on the rim of the nucleus and oral bacteria surrounding the cheek cells.