LIG4 and lymphoma: Knockout models of the gene LIG4, located in human chromosome 13 and consisting of a single long exon, are lethal to mouse embryos because of massive neuronal apoptosis (4); all known human mutations are thus hypomorphic and result in a wide clinical spectrum, ranging from normal to severely compromised immune system with microcephaly, growth failure, facial dysmorphism, mental retardation, hypogonadism, progressive bone marrow failure, and leukemia or lymphoma.