CD40LG and X-linked hyper-IgM syndrome: For example, the mutation in the CD40L gene (resulting in X-linked hyper-IgM syndrome or CD40L deficiency) tends not to affect females, while in males it causes lower levels of IgG, IgE, and IgA (and often higher levels of IgM), neutropenia, impaired NK and T cell cytotoxicity, and makes them suffer from recurrent and opportunistic infections (77).