T-cell large granular lymphocytic leukemia is characterized by clonal expansion of a CD3<sup>+</sup>/CD57<sup>+</sup> subpopulation, which are typically CD8<sup>+</sup> positive cytotoxic T- cells, and can only be diagnosed if there is a persistent, greater than 6 months, elevation of LGL in the blood (usually 2-20 × 10<sup>9</sup>/L), in the absence of an identifiable cause. The gene discussed is B3GAT1; the disease is T-cell large granular lymphocyte leukemia.