Pair-wise comparisons showed that large numbers of miRNAs are differentially expressed between any given two ages. In addition, the number of differentially expressed miRNAs as well as the value of the average fold changes varied between the three developmental ages investigated. As shown in Table 3, the number of differentially expressed miRNAs between E33 and E65 is much smaller than between E65 and Adu, and the value of the average fold change between E33 and Adu is much lower than between E65 and Adu. These findings show that the expression patterns of the three ages are unique. Of the three miRNAs reported as regulators of development in skeletal and BI-D1870 S6 Kinase? inhibitor cardiac muscle, miR-206 was found to be upregulated 2.9-fold in Adu compared to E65, but the expression variance of miR-1 and miR-133 failed to reach statistically significant levels. These two miRNAs showed a high level of expression in the microarray analysis, thus technical error could be ruled out. It should be noted that the functional discovery of these miRNAs was made mostly in cell culture systems, which may differ from the in vivo system. Several of the differentially expressed miRNAs identified here were shown to play roles in growth and development related processes in recent studies. These include miR-214, miR-140, miR-150, miR-10, as well as miR-181. In the zebrafish, miR-214 can modulate hedgehog signaling, thus changing muscle cell fate, and miR-10 was shown to represses HoxB1a and HoxB3a, which are involved in patterning the anterior-posterior axis. In mouse cells, the cartilage specific miRNA, miR-140, targets the histone deacetylase 4, suggestive of a role in long bone development. In mature B and T cells, the miR-150 was found to block early B cell development when expressed prematurely, and also found to control B cell differentiation by targeting the transcription factor of c-Myb. Furthermore, miR-181 was found to be involved in the process of mammalian skeletal-muscle differentiation, by targeting the homeobox protein Hox-A11 during mammalian myoblast differentiation. These findings suggest that identifying differentially expressed miRNAs may lead to the discovery of miRNAs related to muscle growth and development. To visually illustrate the expression type of the miRNAs being expressed during different developmental stages, a hierarchical cluster analysis was performed for the differentially expressed miRNAs. The results show that the miRNA expression patterns fall into seven typical categories: A) prenatally expressed, expression level increased between E33 and E65; B) universally expressed, expression level decreased between E33 and E65; C) universally expressed, expression level increased through the three ages; D) moderately expressed in E65, expression levels in E33 and Adu Reversine nearly undetectable; E) moderately expressed in E33, expression levels in E65 and Adu nearly undetectable; F) postnatally expressed, expression nearly undetectable in prenatal ages; G) moderately expressed, expression level increase through the three ages.