Molecules built of RNA have been the subjects of numerous studies, which have made known new functions and structures that these molecules can create. In recent years, thanks to next-generation sequencing, it is possible to observe very small RNAs and the number of newly discovered RNA molecules is rapidly increasing. Among other small oligonucleotides, structures derived from tRNA and snoRNA molecules have been observed, and these molecules were determined to not be precursors of known RNA molecules. These structures have attracted the attention of researchers because the level of accumulation of tRNA or snoRNA fragments was relatively high. Additionally, other parts of the parent molecules were absent. Derivatives of well-known RNA molecules also have functions that are different from their parent molecules. They are mainly involved in regulating the expression of genetic information in a similar way to miRNA. In addition, some of the miRNAs that have been described are derivatives of tRNA or snoRNA. Most of the research on these newly discovered molecules is based on their detection and on the study of the macro effects that they exert, in the absence of a description of the molecular mechanism by which they arise and work.