RNAs are modified via editing of the sequence and/or modification of bases. The RNA transcript that is directly copied from the DNA is called the primary transcript or the transcription unit to distinguish it from the mature RNA. Most of these post-transcriptional processes on the RNA occur in the nucleus and appear to be regulatory. Following processing, RNAs are transported from the nucleus to the cytoplasm. The process of transport out of the nucleus may also be regulated (see slide "mRNP Transport Through the Nuclear Pore Complex [NPC]"). The stability of the RNA in the cytoplasm also determines how much protein an individual mRNA can produce. The processing and alternative splicing of the RNAs, transport out of the nucleus, cytoplasmic localization, mRNA stability including the phenomenon of RNA interference (RNAi), and regulation of their translation are all forms of differential gene expression that can be described as post-transcriptional gene regulation.