It means that except targeted mutation (TM ), there exits non targeted mutation (NTM), which is the result of genetic instability induced by DNA damaging agents.

• 即DNA损伤剂除了能直接作用于细胞遗传物质而造成损伤部位的定标性突变(targeted mutation，TM)外，还可能通过诱导某些基因的表达或功能改变导致DNA复制差错，从而在未损伤部位形成非定标性突变(non targeted mutation，NTM)。