The marginal improvements in B cell function observed in BLT mice following PC-KLH immunization are reminiscent of both immunized CVID patients who may develop IgM, but not IgG, responses to the vaccine antigens [10], [52], [62] and pediatric severe combined immunodeficiency (SCID) patients who receive allogeneic hematopoietic stem cell transplants but do not gain normal B cell function despite multiple and varied antigen stimulations and infections [63]. The gene discussed is CD40LG; the disease is severe combined immunodeficiency.