Chromatin in eukaryotic nuclei is thought to be partitioned into functional loop domains that are generated by the binding of defined DNA sequences, named MARs (matrix attachment regions), to the nuclear matrix.We have previously identified B-type lamins as MAR-binding matrix components (M. E. E. Luderus, A. de Graaf, E. Mattia, J. L. den Blaauwen, M. A. Grande, L. de Jong, and R. van Driel, Cell 70:949-959, 1992). Here we show that A-type lamins and the structurally related proteins desmin and NuMA also specifically bind MARs in vitro. We studied the interaction between MARs and lamin polymers in molecular detail and found that the interaction is saturable, of high affinity, and evolutionarily conserved. Competition studies revealed the existence of two different types of interaction related to different structural features of MARs: one involving the minor groove of double-stranded MAR DNA and one involving single-stranded regions. We obtained similar results for the interaction of MARs with intact nuclear matrices from rat liver. A model in which the interaction of nuclear matrix proteins with single-stranded MAR regions serves to stabilize the transcriptionally active state of chromatin is discussed.The current view is that eukaryotic chromatin is divided into topologically constrained loops of tens to hundreds of kilobases. These loops are generated by the binding of specialized DNA sequences to an intranuclear framework, known as the nuclear matrix (4) or nuclear scaffold (37). The loop organization of chromatin may be important not only for the compaction of the chromatin fiber but also for the regulation of gene expression. It has been postulated that each loop represents an independent unit of transcription and replication, being insulated from regulatory influences of neighboring loops (for reviews, see references 7, 15, and 54).