Combined immunodeficiencies (CIDs) are a heterogeneous group of disorders characterized by various gene mutations. The mutations in the STK4 (Serine Threonine Kinase 4) gene, which has a role in the regulation of apoptosis and proliferation, can be a cause of immunodeficiency. In the current paper, we reported a case of identical twin brothers with a novel STK4 mutation, one of whom showed clinical manifestations associated with this mutation with a delay of two years. The mutation in the STK4 gene was identified employing Whole Exome Sequencing (WES), and we described the probable reasons for this delay. We found that the STK4 genetic defect caused almost the same clinical symptoms of immunodeficiency in the twin brothers. Meanwhile, the severity of the disease was higher in one of them, which may be due to extra genetic defect in LRBA, and likely differences in the percentage of B lymphocyte population and CD4+/ CD8+ state.