For transgenic cell lines and transgenic animals generated via many methods, the integration site is random and in most cases not known. Integration of a transgene can disrupt an endogenous gene, potentially interfering with interpretation of the phenotype. In addition, knowledge of where the transgene is integrated is important for planning of crosses between animals carrying a conditional allele and a given Cre allele in case the alleles are on the same chromosome. When the transgene insertion site is unknown, zygosity is determined by expensive quantitative PCR-based approach. These limitations often force investigators to maintain transgenic models or cell lines in a hemizygous state, which may lead to less than desired expression levels of the transgene and make it less efficient and more costly.
