Mutations in filamin B (FLNB), a gene encoding a cytoplasmic actin-binding protein, have been found in human skeletal disorders, including boomerang dysplasia, spondylocarpotarsal syndrome, Larsen syndrome, and atelosteogenesis phenotypes I and III. To examine the role of FLNB in vivo, we generated mice with a targeted disruption of Flnb. Fewer than 3% of homozygous embryos reached term, indicating that Flnb is important in embryonic development. Heterozygous mutant mice were indistinguishable from their wild-type siblings. Flnb was ubiquitously expressed; strong expression was found in endothelial cells and chondrocytes. Flnb-deficient fibroblasts exhibited more disorganized formation of actin filaments and reduced ability to migrate compared with wild-type controls. Flnb-deficient embryos exhibited impaired development of the microvasculature and skeletal system. The few Flnb-deficient mice that were born were very small and had severe skeletal malformations, including scoliotic and kyphotic spines, lack of intervertebral discs, fusion of vertebral bodies, and reduced hyaline matrix in extremities, thorax, and vertebrae. These mice died or had to be euthanized before 4 weeks of age. Thus, the phenotypes of Flnb-deficient mice closely resemble those of human skeletal disorders with mutations in FLNB.chondrocytes ͉ endothelial cell ͉ gene targeting F ilamins are large actin-binding proteins that stabilize the actin cytoskeleton, link the actin network with cellular membranes, and mediate interactions between actin and transmembrane receptors (1). In mouse, there are three filamin genes, Flna, Flnb, and Flnc. Flna and Flnb, which encode large proteins with 70% structural similarity, are ubiquitously expressed, whereas Flnc, encoding a much smaller protein, is expressed in heart and skeletal muscles. It has been proposed that filamins are important for fetal development by regulating the communication between extracellular signals and the cellular cytoskeleton to guide migration of cells into appropriate anatomical sites (1). In addition to actin, filamins are capable of binding to a wide range of molecules involved in cellular signaling and transcriptional regulation (2, 3).FLNB was first isolated as a protein that interacts with the cytoplasmic tail of glycoprotein Ib␣ (4, 5). The 2,603-aa chain of FLNB contains an amino-terminal actin-binding domain and a backbone of 24 Ig-like rod domain repeats disrupted by two hinge regions (6). Mouse Flnb is located on chromosome 14 and consists of 47 exons. In mouse embryos, Flnb is expressed in vertebral bodies, and it has been suggested that Flnb may play a role in vertebral segmentation, joint formation, and endochondral ossification (7).Mutations in the human FLNB gene have been found in several skeletal disorders, including spondylocarpotarsal syndrome, autosomal-dominant Larsen syndrome, atelosteogenesis I and III (7), and boomerang dysplasia (8). The skeletal disorders caused by mutations in FLNB are very similar to those caused by mutations in FLNA (9). Interesting...