Monosomy 7 is generally considered as an acquired cytogenetic abnormality within hematopoietic cells, and indicates an especially high risk of progression to bone marrow failure, myelodysplastic syndrome (MDS) or juvenile myelomonocytic leukemia (JMML). We report a case of a 6-month-old female infant with mosaic monosomy 7 who presented with clinical and laboratory evidences of immunodeficiency. The patient had suffered from recurrent respiratory infections since she was born. Peripheral blood lymphocyte subsets revealed an extremely low level of CD19⁺ B lymphocytes (0.3∼0.8%, normal range: 6.4∼22.6%) and a decreased CD4/CD8 ratio (0.67∼1.12, normal range: 1.4∼2.0). Decreased serum levels of IgG (1.53 g/L, normal range: 4.09∼7.03 g/L), IgA (0.10 g/L, normal range: 0.21∼0.47 g/L) and IgM (0.26 g/L, normal range: 0.33∼0.73 g/L) were detected, while complements were normal. Excepting transient neutropenia, routine blood tests were within normal limits. Clinical exome sequencing identified a de novo mosaic monosomy 7, while no pathogenic mutation associated with immunodeficiency was detected. However, peripheral blood cytogenetic analysis was failure to detect monosomy 7 due to the very few cell mitosis. Subsequent fluorescence in situ hybridization (FISH) identified a mosaic monosomy 7 in 58 cells within a total number of 100 cells, which was consistent with clinical exome sequencing. Therefore, the patient was diagnosed with primary immunodeficiency disease (PID) due to mosaic monosomy 7. Intravenous treatment with multiple antibiotic agents and infusion of gamma globulin could control the patient’s respiratory infections effectively. A better understanding of PIDs will enable effective treatments and prevention of infections in these patients.

7 号单体通常被认为是造血细胞内的获得性细胞遗传学异常，表明进展为骨髓衰竭、骨髓增生异常综合征 (MDS) 或幼年型粒单核细胞白血病 (JMML) 的风险特别高。我们报告了一例 6 个月大的女婴，她患有 7 号镶嵌单体，她有免疫缺陷的临床和实验室证据。该患者自出生以来就患有反复呼吸道感染。外周血淋巴细胞亚群显示极低水平的 CD19 ⁺B淋巴细胞（0.3∼0.8%，正常范围：6.4∼22.6%）和CD4/CD8比值降低（0.67∼1.12，正常范围：1.4∼2.0）。IgG（1.53 g/L，正常范围：4.09∼7.03 g/L）、IgA（0.10 g/L，正常范围：0.21∼0.47 g/L）和IgM（0.26 g/L，正常范围： 0.33∼0.73 g/L)，而补体正常。除了暂时性中性粒细胞减少症外，常规血液检查均在正常范围内。临床外显子组测序确定了一个 de novo 镶嵌单体 7，而没有检测到与免疫缺陷相关的致病突变。然而，由于细胞有丝分裂非常少，外周血细胞遗传学分析未能检测到7号单体。随后的荧光原位杂交 (FISH) 在 100 个细胞总数中的 58 个细胞中鉴定出镶嵌单体 7，这与临床外显子组测序一致。所以，患者被诊断为7号镶嵌单体导致的原发性免疫缺陷病（PID）。静脉应用多种抗生素治疗，并输注丙种球蛋白，可有效控制患者的呼吸道感染。更好地了解 PID 将有助于有效治疗和预防这些患者的感染。