Dyggve-Melchior-Clausen syndrome and Smith-McCort dysplasia are recessive spondyloepimetaphyseal dysplasias caused by lossof-function mutations in dymeclin (Dym), a gene with previously unknown function. Here we report that Dym-deficient mice display defects in endochondral bone formation similar to that of DyggveMelchior-Clausen syndrome and Smith-McCort dysplasia, demonstrating functional conservation between the two species. Dymmutant cells display multiple defects in vesicle traffic, as evidenced by enhanced dispersal of Golgi markers in interphase cells, delayed Golgi reassembly after brefeldin A treatment, delayed retrograde traffic of an endoplasmic reticulum-targeted Shiga toxin B subunit, and altered furin trafficking; and the Dym protein associates with multiple cellular proteins involved in vesicular traffic. These results establish dymeclin as a novel protein involved in Golgi organization and intracellular vesicle traffic and clarify the molecular basis for chondrodysplasia in mice and men.bone formation ͉ gene trap mutation ͉ Golgi ͉ proteomics ͉ growth plate M ost mammalian growth occurs ex utero and requires extensive endochondral bone formation, a spatially organized process orchestrated by proliferating growth plate chondrocytes that subsequently differentiate and undergo apoptosis and ossification (1, 2). Inherited defects in bone formation, or osteochondrodysplasias, are responsible for more than 200 clinically distinct diseases that occur with a cumulative incidence exceeding 1 in 10,000 live births (3, 4) caused by mutations in processes critical for osseous bone formation and maintenance. These include growth factor signaling (5), extracellular matrix synthesis (6-13) and remodeling (14-16), protein sorting (17)(18)(19)(20), and vesicle traffic (21,22).Dyggve-Melchior-Clausen syndrome (DMC) and SmithMcCort dysplasia (SMC) are recessive spondyloepimetaphyseal dysplasias caused by loss-of-function mutations in dymeclin (Dym) (23-25). However, the functions of the DYM gene and the molecular mechanisms responsible for DMC and SMC have not been determined. Aside from potential membrane-spanning sequences, N-myristoylation, and protein trafficking (dileucine) motifs (26), the 74-kDa DYM protein lacks features that would guide predictions as to its functions (23,25). In addition to being structurally unique, DYM protein sequences are shared only by interspecies orthologues, and no related sequences are present in the Saccharomyces cerevisiae genome.The present study used genetic and proteomic strategies developed to analyze genes, like Dym, that have been identified by genome sequencing and whose functions are unknown (27)(28)(29). In the present study, dymeclin functions were analyzed starting with a mutation induced by gene trapping in murine ES cells (30) and subsequently introduced into the mouse germline. Dym-deficient mice developed chondrodysplasia similar to that of DMC and SMC, providing a murine model of the human diseases and demonstrating functional conservation between the mouse and ...