Fig. 6. Cell-autonomous activation of Krox20 by Hoxb1. (A-C) Flat mount, at the level of the mid-hindbrain boundary, of a chick hindbrain electroporated with a construct expressing HA-tagged Hoxb1, in situ hybridized for Krox20 and immunolabelled for Hoxb1-HA: (A) Krox20 staining, (B) anti-HA immunofluorescence and (C) merge. (D-F) Transverse sections, at the level of the forebrain-midbrain, of 5-somite stage vhnf1^-/- embryos co-injected with meis1.1 and hoxb1aMyc RNAs, in situ hybridized for krox20 and immunolabelled for Hoxb1aMyc: (D) krox20 staining, (E) anti-Myc immunofluorescence and (F) merge. (G) ChIP of Hoxb1a-bound element C. Chromatin extracted from zebrafish embryos co-injected with meis1.1 and Myc-tagged hoxb1a RNAs was subjected to the ChIP procedure in the absence (first lane) or presence (second lane) of anti-Myc antibody. Immunoprecipitated DNA was analyzed by PCR using primers designed to amplify the core of element C (297 bp) or an unrelated sequence located -22.4 kb upstream of the transcription start site (422 bp). The third lane shows the PCR products obtained from the input chromatin.