Given the partial phenotypic overlap of functional NIK deficiency and IL21 (receptor) deficiency27, 28 (for example, colitis, susceptibility to Cryptosporidium infection, hypogammaglobulinemia and decreased frequencies of class-switched B cells, defective antigen-specific T-cell proliferation and impaired NK-cell cytotoxicity), we sought to exclude an involvement of NIK in IL21-mediated signalling in B cells. The gene discussed is IL21; the disease is cryptosporidiosis.