TYK2 and Immunodeficiency: In this study, we present two cases of patients who had immunodeficiency associated with novel heterozygous mutations in the four-point-one, ezrin, radixin, moesin (FERM) domain region of TYK2. Unlike previous reports on TYK2 deficiency, the patients exhibited severe T-cell lymphopenia characterized by low naïve CD4+ T-cell counts.