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]. The gene discussed is CD27; the disease is activated PI3K-delta syndrome.