Fig. 3. The apical to basal wave of p27^Kip1 expression is regulated at the transcriptional level. (A) Schematic of the modified p27^Kip1/GFP BAC reporter showing the position of the IRES-GFP relative to exon 1 and exon 2 of the p27^Kip1 mouse gene. BAC ends are located 51 kb 5' and 78 kb 3' to the start site of p27^Kip1 transcription. The first exon of the p27^Kip1 gene was removed to avoid overexpression of p27^Kip1. See Materials and methods for further details. (B,C) Whole-mount preparations of the developing cochlear duct from E12.5 to E14.5 were photographed to reveal the endogenous p27^Kip1/GFP transgene (C, green), and then labeled with antibody to p27^Kip1 (B) to demonstrate the correspondence between the pattern of gene and protein expression at each age examined. In E12.5, apex and base are marked and dotted lines indicate the region of initiation of p27^Kip1 and GFP expression. (D,E) Cross-section through the approximate middle cochlea stained to reveal anti-p27^Kip1 (red) and expressing native GFP reporter (green) as above, to show the coincidence of p27^Kip1-GFP transgene expression with p27^Kip1 protein. Scale bar: 100 µm.