It is a small RNA molecules that interfere with gene expression.

miRNA (micro RNA)

Explanation:

The small RNA molecule that interfere with gene expression are miRNA. The miRNA is micro RNA that are non-coding molecules of RNA. These small RNA regulate gene expression after transcription (post transcription).

The miRNA binds to 3' UTR region (untranslated region) and halt the process of protein production and thereby silencing mRNA.