to the nuclear matrix nucleic acids research 1997 vol 25 no 7 1419 1425 matrix attachment regions have been categorized as constitutive permanent or facultative cell type specific 2 the constitutive mars occur in all types of cells irrespective of the tissue in which they are found in contrast the presence of a facultative mar is tissue specific and its use is governed by that tissue mars have been experimentally defined for several gene loci including the chicken lysozyme gene 5 human interferon b gene 6 human b globin gene 7 chicken a globin gene 8 p53 9 and the human protamine gene cluster 10 the chicken lysozyme locus is regulated by a set of well characterized cis regulatory elements each responsible for a distinct subaspect of tissue specificity of expression 27 33 3 transcriptional activation by a matrix associating region binding protein the journal of biological chemistry vol 276 no 24 issue of june 15 pp 21325 21330 2001 transgenic studies have demonstrated that high level tissue specific expression is only seen when the core is present in context of the mars 8 this effect requires the core because mars alone could not produce high level expression although the mars had previously been implicated in negative regulation of the ig locus in non b cells 4 9 12 this was the first demonstration that the mars were required for proper expression in b.