Technical Abstract: MicroRNA (miR) are a class of small RNAs that regulate gene expression by inhibiting translation of protein encoding transcripts through activation of a specific cellular pathway. The small RNA classified as miR are short sequences of 18-26 nucleotide long, encoded by nuclear genes with distinctive properties that comprise 1-5% of known genes. During processing from the primary transcript, the mature miR sequence is loaded into an RNA:protein complex known as the ¿RNA induced silencing complex¿. The sequence of the miR loaded in the complex targets the RNA induced silencing complex to specific binding sites in the 3¿ untranslated region of messenger RNA (mRNA) transcripts, resulting in degradation of the miR:mRNA complex, blocking of initiation, or translocation to processing bodies. In each case, association of RNA induced silencing complex with mRNA causes decreased translation of the targeted gene product. Approximately 40% of genes have transcripts that are potential targets for miR, suggesting that miR play an important role in multiple cellular processes. A single miR can target numerous distinct mRNA for decreased translation, and as a result miR appear to be intimately involved in developmental decisions. Implication in such a wide array of cellular processes has increased interest in evaluating miR in multiple biological models including production livestock.