Affected mice, but not control or unaffected EB mice, exhibited a uniform population of enlarged lymphoblastic cells in the peripheral blood (Figure 3b, far left panel) which displayed an early developmental B cell phenotype (B220loIgM–, Figure 3b 3rd panel from left) and replaced populations of mature IgM+ B cells present in peripheral tissues of unaffected EB and control mice (Figure 3b, second and fourth panels from left). The gene discussed is CD40LG; the disease is epidermolysis bullosa.