The purpose of the invention is to provide means with which it is possible to efficiently select a vector to which a foreign gene has been introduced when a foreign gene is to be introduced by homologous recombination to a vector having multiple sequences homologous with one another. The vector comprises, in succession, a replication origin, a sequence A, a marker gene X, two sequences C and D for introducing a foreign gene by homologous recombination, and a sequence B homologous with sequence A. The two sequences C and D are directly or indirectly adjacent to one another. The vector is used for introducing a foreign gene between the two adjacent sequences C and D.

Disclosed are methods of genetic engineering to manipulate gene copy number in vivo, as well genetic constructs for amplifying gene copy number in vivo, and recombinant cells that comprise amplified genes. The methods of increasing gene copy number involve reducing expression levels of a haploinsufficient gene in the genome of recombinant cells, such as through replacing the endogenous promoter with a weaker promoter.