Background and aimsThere is an increased risk of lymphomas in inborn errors of immunity (IEI); however, germline genetic testing is rarely used in oncological patients, even in those with early onset of cancer. Our study focuses on a child with a recombination-activating gene 1 (RAG1) deficiency who was identified through a screening program for Slavic founder genetic variants among patients who died with malignancy at an early age in Belarus.ResultsWe identified one homozygous founder RAG1 variant out of 24 available DNA samples from 71 patients who developed lymphoma aged