Boys affected with Wiskott-Aldrich syndrome (WAS) present with variable association of thrombocytopenia, eczema and immune deficiency. If untreated, WAS patients may succumb to intracerebral hemorrhages, severe infections or malignancies. Allogeneic bone marrow transplantation (BMT) can cure all aspects of the disease, but HLA-identical donors are not available to all patients and mismatched BMTs are unfortunately associated with high mortality and morbidity. The good success of HLA-matched BMT, however, makes WAS a potential candidate for hematopoietic stem cell gene therapy. WAS patients carry mutations of the Wiskott-Aldrich syndrome protein gene encoding WASP, a 502-amino acid proline-rich protein with demonstrated involvement in the organization of the actin cytoskeleton. To verify the feasibility of genetic correction for this disease, the WASP cDNA was expressed in EBV-immortalized B cell lines obtained from WAS patients using a retroviral vector. Transduced WAS cells showed levels of WASP expression similar to those found in cells from normal donors, without detectable effects on viability or growth characteristics. In addition, retrovirus-mediated expression of WASP led to improvement of cytoplasmic F-actin expression and formation of F-actin-positive microvilli, a process shown to be defective in untransduced WAS cell lines. These preliminary results indicate a potential use for retrovirus-mediated gene transfer as therapy for WAS.