Biotech terminology is often an evolving entity as new discoveries are made and the factors which control synthesis, degradation and the function of certain biomolecules are gradually identified. For this reason, sometimes the difference between certain biomolecules or their functions are unclear until later years, after the names and definitions have come to mean different things to different people. That seems to be the case with siRNA and micro (mi) RNA. The differences between them are subtle, and, depending on the source, the two are essentially the same. However, as we study further and realize there are two distinct sources of small RNA molecules causing RNA interference, and also find subtle differences in how each is made, activated or manifests itself in different organisms, the distinction is becoming more and more clear.