The quantification of ADA activity has been previously performed in sheep lymphoid cells during antibody production after antigenic stimulation (10), peripheral blood cells of patients with hematological disorders, such as lymphoid leukemia and myeloma (11), serum from patients with different liver disorders (12), serum from patients with acquired immunodeficiency virus (HIV) infection (13), synovial fluid from patients with rheumatoid arthritis (14), serum from patients with tuberculous meningitis (15), serum from patients with chronic lymphoid leukemia (16), and other sources. The gene discussed is ADA; the disease is lymphoid leukemia.