An improved promoter and a use thereof. An improvement is to mutate a nucleic acid sequence between 35 region and 10 region in a promoter region into recognition sites for an endonuclease. The improvement can overcome the problem that a strong promoter in a vector based on blue-white screening initiates the transcription or translation of foreign genes and a transcription or translation product might be toxic to a host and cannot be cloned, avoid the deficiency that frameshift mutation of a gene due to a lack of 1-2 bp of the vector at digestion sites results in false positive clones, and eliminate a false negative phenomenon that a plate is rich in blue spots due to a small fragment of foreign DNA and a reading frame of the gene which is unchanged by inserting the foreign DNA.

The invention provides compositions and methods for performing mammalian cell genetics, e.g., genetic screens, using near-haploid cells. The invention further provides genes and gene products isolated using the inventive methods and methods of use thereof.