X chromosome

In the human genome, the X chromosome is one of the 23 chromosomes. It is a submetacentric chromosome, found within group C of the human karyotype. The X chromosome is one of the two sex-determining chromosomes.

The X chromosome is approximately 155 Mbp (million base pairs) in length, comprising ~5% of the haploid genome
The X chromosome has approximately 900-1400 genes.

In humans and all other mammals, the X chromosome is found in both males and females, although usually males have one copy while females have two. Early in the embryonic development of females, one of the X chromosomes in each cell is randomly switched off in a phenomenon called X inactivation (sometimes lyonisation). The random nature of X inactivation means that some cells express the maternal X chromosome while others express the paternal X chromosome. X inactivation is essential in dosage compensation: ensuring that females and males express equal amounts of gene product from the X chromosome. Some of its genes, notably those found in the pseudoautosomal regions, are also found on the Y chromosome; pseudoautosomal regions escape X inactivation.