A construct containing a downstream neomycin resistance gene and Mesp1 cDNA containing the entire coding region positioned at the start site of the Mesp2 gene was inserted into the Mesp2 locus. A non-coding region of Mesp2 was left intact and utilized for RT-PCR analysis, which showed that transcript from the Mesp2 locus was absent, while transcript from both the endogenous and transgenic Mesp1 loci were present.