②腺苷脱氨酶（ADA）缺陷为ADA基因突变，ADA缺乏引起嘌呤旁路和甲基化旁路的中间代谢产物脱氧三磷酸腺苷（dATP）和S-腺苷同型半亮氨酸（S-adenosylhomocysteine）堆积，它们具有细胞毒效应，抑制T、B细胞增殖和分化。
O B-dis I-dis I-dis I-dis I-dis I-dis I-dis I-dis I-dis I-dis I-dis I-dis O B-bod I-bod I-bod O O O O O B-dis I-dis I-dis I-dis I-dis O O B-bod I-bod I-bod I-bod O B-bod I-bod I-bod I-bod I-bod O O O O O O O B-bod I-bod I-bod I-bod I-bod I-bod I-bod O B-bod I-bod I-bod I-bod O O B-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod O B-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod I-bod O O O O O O O O B-bod I-bod O O O O O O O O B-bod I-bod I-bod O O O O O O