Wiskott-Aldrich syndrome (WAS) is a complex primary immunodeficiency disorder associated with microthrombocytopenia, autoinnmunity and susceptibility to malignant lymphoma. At the molecular level, this rare disorder is caused by mutations in the gene encoding the Wiskott-Aldrich syndrome protein (WASP). WASP is a cytosolic adaptor protein mediating the rearrangement of the actin cytoskeleton upon surface receptor signaling. Allogenic hematopoietic stem cell (HSC) transplantation represents a curative approach but remains problematic in light of severe risks and side effects. Recently, HSC gene therapy has emerged as an alternative treatment option. Cumulative preclinical data obtained from WASP-deficient murine models and human cells indicate a marked improvement of the impaired cellular and immunological phenotypes associated with WASP deficiency. The first clinical trial is currently being conducted to assess the feasibility, toxicity, and potential therapeutic benefit of transplanting autologous WASP-reconstituted hematopoietic stem cells.