Blue white screening Screening by insertional inactivation of the lacz gene Lac promoter Ampr Mcs(Multiple cloning sites 多克隆位点) pUC18 lacz (3kb) orl The insertion of a dna fragment interrupts the ORF of lacz" gene, resulting in non-functional gene product that can not digest its substrate x-galBlue white screening Ampr ori pUC18 (3 kb) MCS (Multiple cloning sites, 多克隆位点） Lac promoter lacZ’ Screening by insertional inactivation of the lacZ gene The insertion of a DNA fragment interrupts the ORF of lacZ’ gene, resulting in non-functional gene product that can not digest its substrate x-gal