[1] Severity of symptoms depends on the degree of reduced enzymatic activity of Ligase IV or gene expression.

Ligase IV is a critical component of the non-homologous end joining (NHEJ) mechanism that repairs DNA double-strand breaks.

NHEJ is also used to repair the DNA double-strand break intermediates that occur in the production of T and B lymphocyte receptors.

Some patients have a severe immunodeficiency characterized by pancytopenia, causing chronic respiratory infections and sinusitis.

Patients also have growth retardation and skin conditions, including photosensitivity, psoriasis and telangiectasia.