Of note, it is now known that hypomorphic mutations of WAS cause isolated X-linked thrombocytopenia (XLT),82 which may even be intermittent83; moreover, gain-of-function (GOF) mutations in the GTPase-binding domain of WASp are responsible for isolated X-linked congenital neutropenia.84 This evidence concerns the gene WAS and thrombocytopenia 1.