Fig. 4. Time-course analysis of the activities of elements A, B and C and requirement for Krox20. Transgenic lines carrying chick element A, B or C driving a ß-globin promoter-lacZ reporter were analysed in Krox20 null (A,E,I,M) or wild-type backgrounds (B-D,F-H,J-L,N-P). The embryos were stained with X-gal and the somite stage (ss) or the embryonic age in day post coitum (E) are indicated. (P) Embryo transgenic for construct cC-lacZ analysed by X-gal staining followed by in situ hybridization with a mouse Hoxb1 probe. r, rhombomere; ov, otic vesicle.