X chromosome inactivation escape, illustration. Chromosomes are composed of DNA (deoxyribonucleic acid) and contain sections, called genes, which encode the bodys genetic information. The X chromosome is one of two human sex chromosomes. Females have 2 X chromosomes and males an X and a Y chromosome. In females one of the X chromosomes is inactivated to prevent the genes being expressed twice. The inactivation occurs via Xist (X-inactive specific transcript, red single helix) RNA (ribonucleic acid), which coats the chromosomes DNA. However, around a quarter of the DNA escapes this inactivation. The mechanism for this escape is unknown, but may involve the transcription factor YY1 (orange in top right insert) and CCCTC-binding factor (red in top right insert) or be mediated by topologically associated domains (TADs, bottom right insert), which are regions of the chromosome that physically interact with each other.