It was suggested that exogenous dna fragments ( mainly non - coding sequence ) had new mutagenetic mechanism as well as insertional mutagenesis . in t1 and t2 , a number of mutants were obtained 對這種現象的分析表明,外源全dna片段(主要是非編碼序列)對轉化受體除了插入誘變的作用外,還可能存在一種新的誘變機制。