The 2 . 3kb fragment was then cloned to pbluescript ii sk ( + ) . the resulting plasmid , phz1754 , was subjected to progressive unidirectional deletions using exonucleaseiii provided in the erase - a - base system for dna sequencing 其后的亞克隆和功能試驗(yàn)將膽固醇氧化酶基因定位在phz1148的2 . 3kbecori - sal片段上,將該片段克隆到大腸桿菌載體pbluescript sk ( + )上,形成質(zhì)粒phz1754 。