maintains levels critical for normal somite segmentation clock function.

During vertebrate development, the presomitic mesoderm (PSM) periodically segments into somites, which will form the segmented vertebral column and associated muscle, connective tissue, and dermis. The periodicity of somitogenesis is regulated by a segmentation clock of oscillating Notch activity. Here, we examined mouse mutants lacking only or , which we previously demonstrated act redundantly to prevent PSM differentiation. is not required for somitogenesis, but mutants display a range of vertebral defects. We analyzed mutants by quantifying mRNAs fluorescently labeled by hybridization chain reaction within Imaris-based volumetric tissue subsets. These data indicate that FGF4 maintains levels and normal oscillatory patterns. To support our hypothesis that FGF4 regulates somitogenesis through , we demonstrate genetic synergy between and , but not with . Our data indicate that is potentially important in a spectrum of human Segmentation Defects of the Vertebrae caused by defective Notch oscillations.