SPI1 and hyperinsulinemic hypoglycemia, familial, 4: PU.1 deficiency is a primary immunodeficiency disorder caused by heterozygous pathogenic variants in the SPI1 gene, which is cataloged in the Online Mendelian Inheritance in Man (OMIM) database as Autosomal Dominant Agammaglobulinemia 10 (AGM10, OMIM#619707) (1).