Lamins are classified into A- and B- type isoforms. Specifically, four distinct lamin isoforms have been identified: A, B1, B2, and C. Lamin A and C are alternatively spliced products of a single gene, whereas lamin B1 and B2 are encoded by distinct genes. During mitosis, lamins are phosphorylated by p34cdc2 and solubilized prior to chromosome condensation and nuclear breakdown. A direct role for lamins in chromosome condensation is sμggested by the fact that lamin B binds to specific DNA sequence motifs termed matrix attachment regions (MARs). Mars mediate the interaction of chromatin with the nuclear matrix. Importantly, some of the same processes that involve mitotic chromosome condensation and nuclear envelop breakdown are also activated during apoptosis. It appears that chromosome condensation during apoptosis is accompanied by degradation of lamin protein. Lamin degradation occurs prior to DNA fragmentation and this degradation is useful as an apoptotic marker.