of the disassembly event leads to a complete cell cycle arrest. At the end of mitosis, (anaphase, telophase) there is a nuclear reassembly which is highly regulated in time, starting with the association of 'skeletal' proteins on the surface of the still partially condensed chromosomes, followed by nuclear envelope assembly. Novel nuclear pore complexes are formed through which nuclear lamins are actively imported by use of their NLS. This typical hierarchy raises the question whether the nuclear lamina at this stage has a stabilizing role or some regulative function, for it is clear that it plays no essential part in the nuclear membrane assembly around chromatin. Embryonic development and cell differentiation The presence of lamins in embryonic development is readily observed in various model organisms such as Xenopus laevis, the chick and mammals. In Xenopus laevis, five different types were identified which are present in different expression patterns during the different stages of the embryonic development. The major types are LI and LII, which are considered homologs of lamin B1 and B2. LA are considered homologous to lamin A and LIII as a B-type lamin. A fourth type exists and is germ cell specific. In the early embryonic stages of the chick, the only lamins present are B-type lamins. In further stages, the expression pattern of lamin B1 decreases and there is a gradual increase in the expression of lamin A. Mammalian development seems to progress in a similar way. In the latter case as well it is the B-type lamins that are expressed in the early stages. Lamin B1 reaches the highest expression level, whereas the expression of B2 is relatively constant in the early stages and starts to increase after cell differentiation. With the development of the different kinds of tissue in a relatively advanced developmental stage, there is an increase in the levels of lamin A and lamin C. These findings would indicate that in its most basic form, a functional nuclear lamina requires only B-type lamins. DNA replication Various experiments show that the nuclear lamina plays a part in the elongation phase of DNA replication. It has been suggested that lamins provide a scaffold, essential for the assembly of the elongation complexes, or that it provides an initiation point for the assembly of this nuclear scaffold. Not only nuclear lamina associated lamins are present during replication, but free lamin polypeptides are present as well and seem