The lamins form a meshwork of filaments on the inner, or nucleoplasmic, surface of the nuclear envelope. Currently, mammalian species appear to contain at least four major lamins, called A, B1, B2, and C, of which two (A and C) arise by alternative splicing of the same gene and two minor lamins, D and E, whose molecular weight varies from species to species. Nuclei of embryonic and lymphiod cells contain only lamin B1. Mutations affecting A-type lamins have been associated with a variety of human diseases, including muscular dystrophy, cardiomyopathy, lipodystrophy, and progeria, but mutations in B-type lamins have never been identified in humans. Homozygous lamin B1 mutant mice survive embryonic development but died at birth with defects in lung and bone. Cells obtained from mutant lamin B1 mice are defective in differentiation, polyploidy and senesce prematurely.
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