Immunodeficiency, centromeric instability, and facial anomalies syndrome (ICF) is a rare genetic disease characterized by hypogammaglobulinemia, T cell immune deficiency with age, pericentromeric hypomethylation, facial abnormalities, and intellectual disability. This study aimed to investigate the phenotype and immune function of a girl with ICF2, identify her genetic defect, and explore the potential pathogenic mechanisms of the disease. We identified a homologous deletion mutation in this girl, which involves exons 1-5 and part of introns 1 and 6 of the