A novel homozygous pathogenic variant (NM_005914;c.1980G>C;p.R660S) in the MCM4 gene, which was previously associated with immunodeficiency 54 (MIM:609981) (online supplemental material S13), has been found to co-segregate with the disease in a large family (DYS-55) including five unaffected and two affected individuals presenting with adolescence-onset isolated dystonia. Here, MCM4 is linked to primary immunodeficiency with natural-killer cell deficiency and adrenal insufficiency.