Fig. 3. Fgf8^lacZ expression reflects endogenous Fgf8 expression. ß-Gal activity in Fgf8^lacZ/+ embryos at E9.5 (A) is detected in the OT and RV, and areas of normal Fgf8 mRNA expression, such as the brain. ß-Gal activity correlates with the expression of lacZ mRNA seen in whole mount in situ hybridization at E9.5, but very low levels are seen within the OT and RV (B). Fgf8 transcripts are present in the OT at very low levels compared with limb bud controls and HPRT positive controls (C) shown by semi-quantitative RT-PCR analysis at E9.5. These data suggest that Fgf8^lacZ expression accurately reflects Fgf8 mRNA expression and that ß-gal activity is more prominent in OT/RV than either lacZ or Fgf8 mRNA. OT, outflow tract; LB, limb bud; NEG, negative control tissue.