Fig. 3. Mash1 and not Ngn1 is required for the expression of Notch signalling genes in the OE. Probes for Notch ligands delta-like 3 (A-A''), Serrate 1 (B-B'') and Serrate 2 (C-C'') and for the Notch bHLH effector genes Hes1 (D-D'') and Hes5 (E-E'') hybridised to wild-type (A-E), Mash1 mutant (A'-E') and Ngn1 mutant (A''-E'') OE at E12.5. Expression of Ser1 and Hes1 in apical cells and of Dll3, Ser2 and Hes5 in basal cells is abolished in Mash1 mutant OE. In contrast, expression of these genes is unaffected in Ngn1 mutant OE. Note that Hes1 and Ser1 are also expressed in the mesenchyme surrounding the OE. Scale bar, 25 µm.