1 Introduction Clinical heterogeneity is common among thousands of described Mendelian disorders (Chong et al., 2015; Posey et al., 2019). Likewise, for inborn errors of immunity (IEIs) reduced penetrance and variable expressivity are the rule, not the exception (Gruber and Bogunovic, 2020; Aksentijevich and Schnappauf, 2021). Even the severe combined immune deficiencies, the most paradigmatic of examples, are not uniformly apparent in infancy (Dvorak et al., 2023).
