Critical developmental control genes sometimes contain "shadow" enhancers that 
can be located in remote positions, including the introns of neighboring genes 
[1]. They nonetheless produce patterns of gene expression that are the same as 
or similar to those produced by more proximal primary enhancers. It was 
suggested that shadow enhancers help foster robustness in gene expression in 
response to environmental or genetic perturbations [2, 3]. We critically tested 
this hypothesis by employing a combination of bacterial artificial chromosome 
(BAC) recombineering and quantitative confocal imaging methods [2, 4]. Evidence 
is presented that the snail gene is regulated by a distal shadow enhancer 
located within a neighboring locus. Removal of the proximal primary enhancer 
does not significantly perturb snail function, including the repression of 
neurogenic genes and formation of the ventral furrow during gastrulation at 
normal temperatures. However, at elevated temperatures, there is sporadic loss 
of snail expression and coincident disruptions in gastrulation. Similar defects 
are observed at normal temperatures upon reductions in the levels of Dorsal, a 
key activator of snail expression (reviewed in [5]). These results suggest that 
shadow enhancers represent a novel mechanism of canalization whereby complex 
developmental processes "bring about one definite end-result regardless of minor 
variations in conditions" [6].