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 其后的亚克隆和功能试验将胆固醇氧化酶基因定位在phz1148的2 . 3kbecori - sal片段上,将该片段克隆到大肠杆菌载体pbluescript sk ( + )上,形成质粒phz1754 。