X Chromosome: one of the two sex chromosomes, X and Y. See also Y chromosome, sex chromosome.

Xenogeneic (Heterologous): pertaining to individuals of different species.

Xenograft (Heterograft): tissue or organs from an individual of one species transplanted into or grafted onto an organism of another species, genus, or family. A common example is the use of pig heart valves in humans. A graft derived from an animal of a species different from the one receiving the graft.

X-inactivation: a process by which one of the X chromosomes in each somatic cell of females is rendered inactive. This results in a balance in gene expression between in X chromosomal and autosomal genes, which is necessary because males have only one X chromosome.

X-linked: an allele for a trait or disorder that is located on the X chromosome. May be either dominant or recessive.

X-rays: rays emitted by a particular generator will emit a spectrum of energies.