Hox proteins repress appendage formation in the abdomen

The absence of limbs in the abdomen is controlled by Hox genes. (a) The expression of the Distal-less (Dll) gene (red) marks the position of future appendages, expression of the Hox gene Ultrabithorax (purple) marks the position of the abdominal segments A1 through A7, and expression of the engrailed gene (blue) marks the posterior of each segment. (b) Schematic representation of Ubx⁻ embryo showing that Dll expression (red circles) is derepressed in segment A1. (c) Schematic representation of Ubx⁻abd-A⁻ embryo showing that Dll expression (red circles) is derepressed in the first seven abdominal segments.

[(a) Photomicrograph by Dave Kosman, Ethan Bier, and Bill McGinnis; (b and c) Data from B. Gebelein, D. J. McKay, and R. S. Mann, “Direct Integration of Hox and Segmentation Gene Inputs During Drosophila Development,” Nature 431, 2004, 653-659.]