AbstractSummary: The Notch signaling pathway is important in regulating formation and anterior‐posterior patterning of somites in vertebrate embryos. Here we show that distinct segmentation defects are displayed in embryos mutant for the Notch pathway genes Notch1, Lunatic fringe (Lfng), Delta‐like 1 (Dll1), and Delta‐like 3 (Dll3). Lfng‐deficient mice and Dll3‐deficient mice exhibit very similar defects, and marker analysis suggests that progression of the segmentation clock is disrupted in Dll3 mutants. We also show that Radical fringe (Rfng)‐deficient mice exhibit no obvious phenotypic defects. To assess whether the absence of a phenotype in Rfng‐deficient mice was the result of functional redundancy with the Lfng gene, we generated Lfng/Rfng double homozygous mutant mice. These mice exhibit the skeletal defects normally observed in Lfng‐deficient mice, but we detected no obvious synergistic or additive effects in the double mutant animals. genesis 33:21–28, 2002. © 2002 Wiley‐Liss, Inc.