In Wiskott–Aldrich syndrome (WAS) and X-linked thrombocytopenia (XLT), mutations in WAS (encoding WASP) cause microthrombocytopenia (that is, reduced numbers and size of blood platelets), immunodeficiency, eczema, increased malignancies and autoimmune symptoms including vasculitis and inflammatory bowel disease5, 6, 7, 8. Here, WAS is linked to hereditary thrombocytopenia with normal platelets.