Studies in the alymphoplasia (aly/aly) and Nik-knockout mice described B cell deficiency due to disorganized lymph nodes, Peyer’s patches and splenic architecture, accompanied by B cell lymphopenia and low serum Ig levels due to compromised class switch recombination (CSR) and somatic hypermutation (SHM) (8, 9, 11, 13, 24, 29). The gene discussed is MAP3K14; the disease is lymphopenia.