Xenopus laevis (South African clawed toad) Species of frog (not toad)
frequently used in studies of early vertebrate development.

XIC see X-inactivation center

X-inactivation Inactivation of one copy of the X chromosome in the somatic
cells of female mammals.

X-inactivation center (XIC) Site in an X chromosome at which inactivation is
initiated and spreads outwards.

X-ray crystallography Technique for determining the three-dimensional
arrangement of atoms in a molecule based on the diffraction pattern of X-rays
passing through a crystal of the molecule.