Similar deficiency of Treg has been reported in patients with CVID.37, 38, 39, 40 Although some of the molecular defects in CVID have been identified, including mutations of ICOS, TACI and BAFF-R, and CD19, CD20, CD21, CD81, MSH4 and LRBA genes,7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 the genetic basis for this disorder still remains to be elucidated. Here, ICOS is linked to common variable immunodeficiency.