FOXN1 and B cell deficiency: In this context, previous reports have shown that combining a ‘nude’ Foxn1-null genotype or a CD40-null genotype with the Xid genotype severely exacerbates the Xid phenotype of peripheral B cell scarcity32, even though neither the Foxn1-null nor the CD40-null genotype show any B cell deficiency by themselves33, 34.