S/MARs (scaffold/matrix attachment regions) are the DNA regions that are involved in the interaction with the nuclear matrix and are identified by in vitro methods. According to the available information, S/MARs possess an insulating activity, i.e., the ability to block the interaction between the enhancer and promoter in vivo, and are, probably, intact insulators or their fragments. Nevertheless, there is still no direct proof for this correspondence. To obtain additional information on the insulator activity of S/MARs, we selected five DNA fragments of different lengths and affinities for the nuclear matrix from a previously constructed library of S/MARs and tested their ability to serve as insulators. Two of five elements exhibited an insulator (enhancer blocking) activity upon the transient transfection of CHO cells. None of the S/MARs displayed either promoter or enhancer/silencer activities in these cells. © 2005 Pleiades Publishing, Inc.