In human patients, mutations in the cytosolic WASP result in the Wiskott-Aldrich syndrome (WAS), an X-linked primary immunodeficiency that is characterized by a progressive decline in T-cell numbers, failure to produce antibodies to polysaccharide and protein antigens, low levels of serum IgM, and elevated levels of IgE resulting in recurrent infections, thrombocytopenia, gastrointestinal bleeding, eczema, and a predisposition to lymphocyte-mediated malignancies [76,86]. This evidence concerns the gene WAS and Wiskott-Aldrich syndrome.