The study investigates genetic mutations in the lymphocytic variant of hypereosinophilic syndrome (L-HES) in five siblings, of which three are symptomatic and two are asymptomatic. L-HES is a rare subtype of hypereosinophilic syndrome caused by aberrant T-cell clones that promote eosinophilia by overproducing interleukin-5 (IL-5). The index patient (EOS1) had clonal CD3-CD4+ T cells, marked eosinophilia, and papillary thyroid carcinoma. Methods such as flow cytometry, TCR gene rearrangement studies, a targeted NGS panel of 141 genes, and whole exome sequencing (WES) were used to identify rare germline variants. The analysis revealed non-synonymous variants of uncertain significance in the genes ZNF257, MLLT1, BRD9 (transcriptional regulation), TESPA1, LRCH4, DHX58 (immune signaling) and CTAGE4, RGPD5 (oncogenesis). No STAT3 mutations or recurrent mutations were found. Some variants were shared by affected siblings but absent in unaffected controls. The study highlights the need for functional validation and long-term monitoring for risk stratification and detection of malignant transformation.