BACKGROUND: Mutations in the SP110 gene result in infantile onset of the autosomal recessive primary immunodeficiency disease veno-occlusive disease with immunodeficiency syndrome (VODI), which is characterized by hypogammaglobulinemia, T-cell dysfunction, and a high frequency of hepatic veno-occlusive disease. OBJECTIVES: We sought to further characterize the clinical features, B-lineage cellular immunologic findings, and molecular pathogenesis of this disorder in 9 patients with new diagnoses, including 4 novel mutations from families of Italian, Hispanic, and Arabic ethnic origin. METHODS: Methods used include clinical review; Sanger DNA sequencing of the SP110 gene; determination of transfected mutant protein function by using immunofluorescent studies in Hep-2 cells; quantitation of B-cell subsets by means of flow cytometry; assessments of B-cell function after stimulation with CD40 ligand, IL-21, or both; and differential gene expression array studies of EBV-transformed B cells. RESULTS: We confirm the major diagnostic criteria and the clinical utility of SP110 mutation testing for the diagnosis of VODI. Analysis of 4 new alleles confirms that VODI is caused by reduced functional SP110 protein levels. Detailed B-cell immunophenotyping demonstrated that Sp110 deficiency compromises the ability of human B cells to respond to T cell-dependent stimuli and differentiate into immunoglobulin-secreting cells in vitro. Expression microarray studies have identified pathways involved in B-lymphocyte differentiation and macrophage function. CONCLUSION: These studies show that a range of mutations in SP110 that cause decreased SP110 protein levels and impaired late B-cell differentiation cause VODI and that the condition is not restricted to the Lebanese population.