In the work presented here, we elucidate a mechanism for the repression of a-myosin heavy chain (MyHC) during pathological cardiac hypertrophy. We demonstrate that the transcription factor Yin Yang 1 (YY1) significantly decreases endogenous alpha-MyHC mRNA and protein expression in neonatal rat ventricular myocytes. Furthermore, mutation of the YY1 binding sites in the proximal rat alpha-MyHC promoter increases promoter activity and alleviates YY1-mediated repression of the promoter. Despite the presence of 5 sites that bind YY1, only one site, located at -94 bp of the rat alpha-MyHC promoter, is both necessary and sufficient for pathological repression of the promoter by phorbol esters, revealing a unique mechanism for the repression of alpha-MyHC expression during cardiac hypertrophy. (C) 2004 Elsevier Inc. All rights reserved.