Although CRISPR/Cas-mediated mutagenesis can be used to probe immune system function (21), we have so far failed to generate FOXN1 crispants amenable to meaningful immune system analysis, as they were lost early in development, perhaps owing to the ensuing T cell immunodeficiency and/or pleiotropic effects of FOXN1 mutations. The gene discussed is FOXN1; the disease is T-cell immunodeficiency.