LAT deficiency is also associated with abnormal lymphocyte frequency and function, including progressive lymphopenia, reduced CD4+ and CD8+ T cell numbers, expansion of CD4-CD8- double negative T cells, and reduced activation and proliferation of T cells following anti-CD3/anti-CD28 stimulation (6, 7). This evidence concerns the gene LAT and hyperinsulinemic hypoglycemia, familial, 4.