ICOS and common variable immunodeficiency: Genes that have been associated with monogenic CVID include ICOS, NFKB1, NFKB2, IKZF1 (IKAROS), TNFSF12 (TWEAK), TNFRSF13B (TACI), TNFRSF13C (BAFF-R), CTLA4, CD19, CD81, CR2 (CD21), MS4A1 (CD20), TNFRSF7 (CD27), IL21, IL21R, LRBA, PRKCD, PLCG2, PIK3CD, PIK3R1, VAV1, RAC2, BLK, and IRF2BP2 though diseases related to these are now considered distinct clinical entities, as in the case of activated Phosphoinositide 3-kinase δ syndrome (APDS) [17,21].