Inborn errors of immunity (IEIs) are monogenic genetic immune disorders, with over 450 underlying genetic mutations identified. Curiously, there is considerable phenotypic variability among carriers of identical genetic mutations. A study in Nature finds that this can be explained by allelic transcription bias.

Most genes are expressed in a biallelic manner, except X and Y chromosome-linked genes and specific gene families such as the immunoglobulins. In addition, autosomal random monoallelic expression (aRMAE) can cause somatic commitment to stable monoallelic gene expression. Stewart et al. used a clonal primary T cell system to assess the aRMAE status of 189 IEI-associated genes in healthy donors and found that around 4% of these can undergo aRMAE.