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Craniofacial dysmorphisms are among the most common birth defects. Proteasome mutations frequently result in craniofacial dysmorphisms, including lower jaw malformations; however, the underlying mechanisms are unknown. Here, we used a zebrafish proteasome subunit β 1 (psmb1) mutant to define the cellular mechanisms underlying proteasome mutation-induced craniofacial dysmorphisms. psmb1 mutants exhibited a flattened ceratohyal and smaller Meckel's and palatoquadrate cartilages. Ceratohyal flattening was a result of failed chondrocyte convergent extension, accompanied by reduced numbers of chondrocytes in the lower jaw due to defects in chondrocyte differentiation. Morphogenesis of craniofacial muscles and tendons was similarly perturbed. psmb1 mutants lacked the hyohyal muscles, and craniofacial tendons were shortened and disorganized. We additionally identified a critical period for proteasome function in craniofacial development, specifically during chondrocyte and muscle differentiation. psmb1 overexpression in sox10+ cells of mutant embryos rescued both cartilage and tendon phenotypes but induced only a partial rescue of the muscle phenotype, indicating that psmb1 was required in both tissue-autonomous and nonautonomous fashions during craniofacial development. Overall, our work demonstrates that psmb1 is required for craniofacial cartilage, tendon, and muscle differentiation and morphogenesis.