The X chromosome is one of the two sex chromosomes in humans — the other being the Y chromosome — and is present in two copies in biological females (XX) and one copy in biological males (XY). It carries a significant number of protein-coding genes beyond those related to sex determination, including many essential for neurological function, immune response, and metabolic processes. In the context of gene therapy, the X chromosome is of particular clinical relevance because mutations in X-linked genes cause a range of severe inherited diseases — including hemophilia A, Duchenne muscular dystrophy (DMD), Fabry disease, and X-linked severe combined immunodeficiency (X-SCID) — many of which are primary targets for viral vector-based gene therapy programs manufactured by CDMOs.
