meis1-MO injection reduces rhombomeres-3 krox20 expression specifically. (A) Ctrl-MO has no effect on krox20 expression in rhombomeres 3 (r3) or 5 (r5). (B) meis1-MO embryos show a specific reduction of r3 krox20 expression (red arrowhead). (C) Injection of a meis1, 2.2, 3 and 4 MO cocktail results in an even stronger inhibition of r3 krox20 expression (red arrowheads). (D) This effect is partially rescued by injection of GFP-meis1 mRNA (meis1GFP) (black arrows). GFP-meis1 is insensitive to meis1-MO, since the N-terminal tagging of meis1 with GFP destroys the meis1-MO target sequence in the GFP-meis1 mRNA. These results phenocopy the reported effect on r3 krox20 expression caused by injection of a dominant-negative Meis form (Waskiewicz et al., 2001) and thus confirm the biological specificity of the Meis mopholinos used in this study.