The intergenic region between the mouse alpha-cardiac myosin heavy chain and 
beta-myosin heavy chain genes has previously been shown to direct expression of 
the bacterial chloramphenicol acetyltransferase reporter gene in transgenic mice 
in a tissue-specific manner. Sequence analyses located a putative 
myocyte-specific enhancer-binding factor (MEF-2) site situated in the regulatory 
region of this gene proximal to the start site of transcription. The role of 
this element in directing the cardiac compartment-specific expression of the 
transgene was assessed. The polymerase chain reaction was used to perform 
substitution mutagenesis of the MEF-2 binding site, and lack of MEF-2 binding 
was confirmed by gel retardation assays. The resultant construct was used to 
generate transgenic mice. Surprisingly, transgene expression was not 
down-regulated, but was significantly increased in the hearts of the MEF-2 
mutant mice. In addition, cardiac-specific expression of the transgene was 
perturbed with significant levels of ectopic expression occurring in the aorta.