Mutations in a number of genes associated with B-cell development and functions, that is, ICOS, TACI and BAFF-R, and CD19, CD20, CD21, CD81, MSH5 and LRBA have been observed in patients with CVID.7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 However, they contribute to approximately 20% of all cases of CVID. The gene discussed is MSH5; the disease is common variable immunodeficiency.