People with lactose intolerance usually don't have enough lactase to break down the sugar in milk.

lactase 是由 lactose（乳糖）和酶类名词后缀 -ase 组成的复合词，字面意思是"分解乳糖的酶"。在生物化学中，-ase 结尾的单词通常表示某种酶。