NHEJ1 and Immunodeficiency: Next we examined the effect of UPR activation following readthrough treatment in another disease model, a human fibroblast cell line (P133) derived from a patient with immunodeficiency with microcephaly, homozygous for the nonsense mutation R178X in the Cernunnos/XRCC4-like factor (XLF) gene (Buck et al, 2006).