To evaluate a novel candidate disease gene, we engaged international collaborators and identified rare, biallelic, specifically homozygous, loss of function variants in SENP7 in four children from three unrelated families presenting with neurodevelopmental abnormalities, dysmorphism, and immunodeficiency. Their clinical presentations were characterized by hypogammaglobulinemia, intermittent neutropenia, and ultimately death in infancy for all four patients. SENP7 is a sentrin-specific protease involved in posttranslational modification of proteins essential for cell regulation, via a process referred to as deSUMOylation. We propose that deficiency of deSUMOylation may represent a novel mechanism of primary immunodeficiency.