Tehran University of Medical Sciences Acta Medica Iranica 0044-6025 57 11 2020 05 13 678 681 EN Shahnaz Armin Pediatric Infections Research Center, Mofid Children Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. AND Mofid Children’s Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Keyvan Ramezani Mofid Children’s Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Bibi Shahin Shamsian Mofid Children’s Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Zahra Chavoshzadeh Pediatric Infections Research Center, Mofid Children Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. AND Mofid Children’s Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Maryam Eghbali Department of Genetics, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Alireza Zare Bidoki The John Curtin School of Medical Research, Australian National University, Canberra 2601, Australia. AND Network of Immunity in Infection, Malignancy and Autoimmunity (NIIMA), Universal Scientific Education and Research Network (USERN), Canberra, Australia. Maryam Sadr Molecular Immunology Research Center, Tehran University of Medical Sciences, Tehran, Iran. Mehrnaz Mesdaghi Department of Pediatrics, Mofid Children Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran Hooshang Gorjipour Mofid Children’s Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Sepideh Razi 7 Student Research Committee, School of Medicine, Iran University of Medical Sciences, Tehran, Iran. AND Cancer Immunology Project (CIP), Universal Scientific Education and Research Network (USERN), Tehran, Iran. Nima Rezaei Research Center for Immunodeficiencies, Children’s Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran. AND Department of Immunology, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. ANDNetwork of Immunity in Infection, Malignancy and Autoimmunity (NIIMA), Universal Scientific Education and Research Network (USERN), Tehran, Iran. 2018 12 01 2020 04 24 The hyper-immunoglobulin M (HIGM) syndrome comprises a group of rare inherited immunodeficiency disorders characterized by normal or elevated levels of serum IgM with low or absent levels of serum IgG, IgA, and IgE. Patients with this syndrome usually present with a history of recurrent infections or opportunistic infections. Here, we report two male cousins from homozygote twin mothers. The first cousin presented with no signs or symptoms other than neutropenia, which was accidentally found in a routine blood test. Immunological workup in this patient showed undetectable IgG and IgA levels and normal IgM levels. The second cousin had a history of recurrent infections, and at the time of admission, he was diagnosed with Pneumocystis jirovecii infection. The immunologic workup of this patient showed undetectable IgG, decreased IgA, and increased IgM level. Due to their interesting family relationship, genetic analysis was performed, which detected a novel mutation in exon 2 (c.266 del G) of the CD40 ligand gene (CD40LG). https://acta.tums.ac.ir/index.php/acta/article/view/7609 https://acta.tums.ac.ir/index.php/acta/article/download/7609/5345