20 Aruffo Aruffo A A Farrington Farrington M M Hollenbaugh Hollenbaugh D D Li Li X X Milatovich Milatovich A A Nonoyama Nonoyama S S The CD40 ligand, gp39, is defective in activated T cells from patients with X-linked hyper-IgM syndrome The CD40 ligand, gp39, is defective in activated T cells from patients with X-linked hyper-IgM syndrome Cell. The gene discussed is CD40LG; the disease is X-linked hyper-IgM syndrome.