These include BTK for XLA, which is expressed at precise levels at different stages of B cell development and activation and unregulated BTK expression could be oncogenic (83); CD40L for X-linked Hyper-IgM syndrome, which has been shown to be oncogenic if constitutively expression (84, 85); FOXP3 which needs to be expressed in regulatory T cells and their precursors, as well as during T cell activation; and RAG1, as discussed above. Here, BTK is linked to X-linked hyper-IgM syndrome.