  The precise layout of gene expression patterns is a crucial step in
development. Formation of a sharp boundary between high and low expression
domains requires a genetic mechanism which is both sensitive and robust to
fluctuations, a demand that may not be easily achieved by morphogens alone.
Recently it has been demonstrated that small RNAs (and, in particular,
microRNAs) play many roles in embryonic development. While some RNAs are
essential for embryogenesis, others are limited to fine-tuning a predetermined
gene expression pattern. Here we explore the possibility that small RNAs
participate in sharpening a gene expression profile that was crudely
established by a morphogen. To this end we study a model where small RNAs
interact with a target gene and diffusively move from cell to cell. Though
diffusion generally smears spatial expression patterns, we find that
intercellular mobility of small RNAs is actually critical in sharpening the
interface between target expression domains in a robust manner. We discuss the
applicability of our results, as examples, to the case of leaf polarity
establishment in maize and Hox patterning in the early {\it Drosophila} embryo.
Our findings point out the functional significance of some mechanistic
properties, such as mobility of small RNAs and the irreversibility of their
interactions. These properties are yet to be established directly for most
classes of small RNAs. An indirect yet simple experimental test of the proposed
mechanism is suggested in some detail.
