ATP6AP1 and immunodeficiency disease: Here we outline a case of two male siblings (patient 1—P1, patient 2—P2; Figure 1A,B) from a non‐consanguineous Caucasian Czech family with a new pathogenic mutation in ATP6AP1, who presented with a very severe phenotype resulting in early death due to liver failure, while they showed no signs of immunodeficiency or neurologic involvement like the majority of the 13 previously described ATP6AP1‐CDG cases.1, 6, 7 For a comparison of selected clinical and laboratory data, see Table 1; the detailed description of our patients' clinical course is available in Supporting Information.