The nuclear lamina is a dense, ~ 30 to 100 nanometers thick, fibrillar network composed of intermediate filaments made of lamin that lines the inner surface of the nuclear envelope in animal cells. At the onset of mitosis, a certain cyclin-dependent kinase complex (Cdk1 and cyclin B in mammals) phosphorylates the lamins, causing them to undergo a conformational change that triggers the disassembly of the nuclear envelope. After the chromosomes have migrated to each pole, the lamins are dephosphorylated, allowing them to begin to reform the nuclear lamina. Initially the lamina reassembles directly on the outer surface of the condensed chromosomes by binding to proteins which associate the condensed DNA, thus excluding all cytoplasmic proteins from the newly forming intranuclear space. The lamins also bind to lamin receptor proteins embedded in the membrane of the endoplasmic reticulum (the ER membrane is continuous with the nuclear envelope). This allows the nuclear envelope to reform around each of the chromosomes. As the chromosomes unwind, their individual coatings of ER membrane merge with one another to form a single nuclear envelope supported by the nuclear lamina. Even during interphase the lamins maintain interactions with DNA binding proteins or DNA itself, especially with heterochromatin which tends to cluster at the surface of the nuclear lamina.