Flk1 expression in 9.5 dpc embryos carrying the various Flk1 mutant alleles after different reaction times. X-gal staining was performed for 1, 4, and 24 hours using Flk1⁺/lacZ neo-out mice (A-C), Flk1⁺/lacZ neo-in mice (D-F), and Flk1⁺/lacZ del Sw/B neo-out mice (G-I). Note marked increase of lacZ activity in Flk1⁺/lacZ neo-out embryo compared with that of Flk1⁺/lacZ neo-in embryo, and that there is no significant change in lacZ activity regardless of the absence of the first intronic enhancer. (J-M) Surface view of adult brains stained with X-gal for 16 hours isolated from wild-type (J), Flk1⁺/lacZ neo-in (K), Flk1⁺/lacZ neo-out (L), and Flk1⁺/lacZ Sw/B neo-out mice (M). Note that lacZ expression in Flk1⁺/lacZ neo-out mice is stronger than Flk1⁺/lacZ neo-in.