Gain of function mutation was first discovered in patients with chronic mucocutaneous candidiasis (CMC). This disease is characteristic with its symptoms as persistent infections of the skin, mucosae - oral or genital and nails infections caused by "Candida", mostly "Candida albicans". CMC may very often result from primary immunodeficiency. Patients with CMC often suffer also with bacterial infections (mostly "Staphylococcus aureus"), also with infections of the respiratory system and skin. In these patients we can also find viral infections caused mostly by Herpesviridae, that also affect the skin. The mycobacterial infections are often caused by "Mycobacterium tuberculosis" or environmental bacteria. Very common are also autoimmune symptoms like type 1 diabetes, cytopenia, regression of the thymus or systemic lupus erythematosus. When T-cell deficient, these autoimmune díseases are very common. CMC was also reported as a common symptom in patients with hyper immunoglobulin E syndrome (hyper-IgE) and with autoimmune polyendocrine syndrome type I. There was reported an interleukin 17A role, because of low levels of IL-17A producing T-cells in CMC patients.