Here, we confirm and expand on previous observations about the clinical utility of the TRBC1-FCM assay for the rapid and easy assessment of T-cell clonality in the blood of individuals presenting with LGL lymphocytosis who are suspicious of T-LGLL based on the demonstration of altered (increased or decreased) percentages of TRBC1+ Tαβ cells (around either 0 or 100%). This evidence concerns the gene TRBC1 and T-cell large granular lymphocyte leukemia.