Small RNAs play an important role in regulating gene expression in both the cytoplasm and the nucleus through inducing both posttranscriptional and transcriptional gene silencing mechanisms. In additional to RNA interference (RNAi), some studies also show that some small RNAs can increase gene expression via a mechanism called RNA activation (RNAa) [166]. Through these regulatory activities, small RNAs are involved in many cellular processes, affect growth and development, and if their own expression goes awry, lead to diseases such as cancer and Alzheimer’s disease.