tokens: The observation that binding sites for the nuclear factor-mu negative regulator ( NF-muNR ) enhancer repressor overlap nuclear matrix attachment regions ( MARs ) in this enhancer has lead to the hypothesis that the cell type specificity of the enhancer might be controlled by regulating nuclear matrix attachment ( Scheuermann , R. H. , and Chen , U. ( 1989 ) Genes & Dev. 3 , 1255-1266 ) .