Fig. 3. Reduced Notch activity and AP patterning defects in msd::Dll1^dn homozygous embryos. Consistent with reduced Notch activity in msd::Dll1^dn embryos, expression of the Notch target Hes5 was not detected in the presomitic mesoderm of day 9.5 and 11.5 msd::Dll1^dn embryos (C,D) in contrast to wild type (asterisks in A,B). Uncx4.1 expression (E-H) in posterior somite halves was significantly downregulated in transgenic embryos (arrowheads in H), whereas expression of the anterior somite compartment marker Tbx18 (I-L) was expanded into posterior somite halves (bracket in L) of day 9.5 embryos. Differential expression of the sclerotome marker Pax9 in anterior and posterior somite compartments (M-P) was less distinct in msd::Dll1^dn day 9.5 embryos particularly in the cervical region (N,P).