X-linked lymphoproliferative (XLP) syndrome Rare immunodeficiency diseases that result from mutations in the gene SH2D1A (XLP1) or XIAP (XLP2). Boys with this deficiency typically develop overwhelming Epstein–Barr virus infection during childhood, and sometimes lymphomas.
An immunodeficiency disease in which T-cell development fails at an early intrathymic stage and no production of mature T cells or T-cell dependent antibody occurs. It is due to a defect in a gene that encodes the γc chain shared by the receptors for several different cytokines.
XLP See X-linked lymphoproliferative syndrome.
XRCC4 A protein that functions in NHEJ DNA repair by interacting with DNA ligase IV and Ku70/80 at double-strand breaks.
ζ chain   One of the signaling chains associated with the T-cell receptor that has three ITAM motifs in its cytoplasmic tail.
ZAP-70 (ζ-chain-associated protein) A cytoplasmic tyrosine kinase found in T cells that binds to the phosphorylated ζ chain of the T-cell receptor and is a key enzyme in signaling T-cell activation.
ZFP318 A spliceosome protein expressed in mature and activated B cells, but not immature B cells, that favors splicing from the rearranged VDJ exon of immunoglobulin heavy chain to the Cδ exon, thereby promoting expression of surface IgD.
zoonotic Describes a disease of animals that can be transmitted to humans.
zymogens An inactive form of an enzyme, usually a protease, that must be modified in some way, for example by selective cleavage of the protein chain, before it can become active.