Fig. 5. Dll1-Notch signaling consists of both Psen1-dependent and Psen1-independent pathways. Normal Uncx4.1 expression (A) is lost in both Dll1 (B) and Psen1-null (C) embryos, as well as in Dll1/Psen1 double-null embryo (D). The stripe expression of the rostral marker Cer1 (E) is almost lost in the Dll1-null embryo (F), whereas it is expanded in the Psen1-null embryo (G). This expanded expression is lost by the additional loss of Dll1 (the Dll1/Psen1 double-null embryo, H). Likewise, Mesp2 expression (I) is almost lost in the Dll1-null embryo (J), moderately reduced in the Psen1-null embryo (K) and is almost lost in the Dll1/Psen1-double null embryo (L). (M-P) Mesp2 expression is partly dependent on Psen1-dependent Dll3-Notch signaling. When compared with the wild type (M), expression levels of Mesp2 are decreased in the Dll3-null (N), Psen1-null (O) and Dll3/Psen1 double-null (P) embryos, and they are comparable among the three genotypes.