Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

xeroderma pigmentosum A rare inherited disease due to inactivating mutations in any of seven genes encoding proteins involved in DNA nucleotide excision repair. Patients are abnormally sensitive to the UV in sunlight and have a high incidence of multiple types of skin cancer.

x-ray crystallography Commonly used technique for determining the three-dimensional structure of macromolecules (particularly proteins and nucleic acids) by passing x-rays through a crystal of the purified molecules and analyzing the diffraction pattern of discrete spots that results. (Figure 3-45)

zinc finger Several related DNA-binding structural motifs composed of secondary structures folded around a zinc ion; present in numerous eukaryotic transcription factors. (Figures 3-10c and 9-30a and b)