Increased microhomology at the CSR junctions has also been observed in mice deficient in PMS2, MLH1, MSH4 or MSH5 and in IgAD and common variable immunodeficiency (CVID) patients carrying mutations in the MSH5 gene (Ehrenstein et al. 2001; Schrader et al. 2002; Sekine et al. 2007; Pan-Hammarstrom & Hammarstrom 2008). The gene discussed is MSH4; the disease is common variable immunodeficiency.