A full-length cDNA encoding human E1A binding protein p300 was ligated to the 5' end of a 5.5-kb fragment of mouse genomic DNA that includes the entire, promoter-containing intergenic region between the beta- (downstream) and alpha-cardiac myosin heavy chain genes. RT-PCR with transgene-specific primers detected mRNA expression in heart, but not in lung, liver or kidney. Immunoblot analysis detected 8-9-fold as much of the (endogenous plus transgene-encoded) protein in cardiac nuclear extracts from transgenic than from wild-type mice.