Firtina, SinemSaritas, MerveNg, Yuk YinNepesov, SerdarKiykim, AycaBozkurt, SelcenSayitoglu, Muge2025-09-252025-09-2520250257-277X1559-0755https://doi.org/10.1007/s12026-025-09638-1https://hdl.handle.net/20.500.12573/3966Bozkurt, Selcen/0000-0002-1158-2072; Kiykim, Ayca/0000-0001-5821-3963; Bilgic Eltan, Sevgi/0000-0003-0561-3343;Severe combined immunodeficiency (SCID) represents a life-threatening inborn error of immunity, necessitating rapid diagnosis and intervention to prevent fatal outcomes. While SCID is characterized by profound T-cell lymphopenia, it may overlap with other conditions like ataxia-telangiectasia (AT), which also presents with T-cell deficiencies. This study examines two cases of suspected SCID in infants, later identified as AT due to pathogenic variants in the ATM gene. Despite initial negative results from SCID-targeted gene panels, further genetic testing revealed nonsense mutations (p.Y2036X and p.E1996X) in the FAT domain of the ATM gene, confirmed by Sanger sequencing. The patients exhibited significant T-cell lymphopenia and reduced ATM protein activity, indicative of AT. These findings highlight the importance of comprehensive genetic screening beyond common SCID-associated genes, especially in patients with atypical presentations. Early and accurate diagnosis can prevent mismanagement and guide appropriate therapies, improving patient outcomes.eninfo:eu-repo/semantics/closedAccessAtaxia-TelangiectasiaSevere Combined ImmunodeficiencyGenetic DiagnosisT-Cell LymphopeniaAtm GeneArticle10.1007/s12026-025-09638-12-s2.0-105005397920