On the contrary, low frequencies of CD4+CXCR5+ cells have been described in several monogenic PIDs like ICOS deficiency (22), X-linked agammaglobulinaemia (23), STAT3 deficiency (24), and also IL10R, CD40L, and NEMO mutations (25). Here, CXCR5 is linked to hyperinsulinemic hypoglycemia, familial, 4.