We don't have synonyms for microrna yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are micron, micro rna and micro-rna.
(n) :
(genetics) A single-stranded, non-coding form of RNA, having only about 20-30 nucleotides, that has a number of functions including the regulation of gene expression
(n) :
(genetics) A single-stranded, non-coding form of RNA, having only about 20-30 nucleotides, that has a number of functions including the regulation of gene expression