A requirement for beta4galactosyltransferase-1 (beta4GalT-1) activity in the modulation of Notch signaling by the glycosyltransferase Fringe was previously identified in a mammalian co-culture assay. Notch signaling is necessary for the formation of somites in mammals. We therefore investigated the expression of eleven Notch pathway and somitogenic genes in E9.5 mouse embryos lacking beta4GalT-1. Four of these genes were altered in expression pattern or expression level. The Notch target genes Hes5 and Mesp2 were affected to some degree in all mutant embryos. The Notch ligand genes Dll1 and Dll3 were reduced or altered in expression in a significant proportion of mutants. While there were no differences in the number or morphology of somites in E9.5 B4galt1 null embryos, the number of lumbar vertebrae in mutant embryos differed from control littermates (P < or = 0.01). The subtlety of the in vivo phenotype may be due to redundancy since several B4galt genes related to B4galt1 are expressed during embryogenesis.