Now that we’ve seen how pre-mRNAs are processed into mature, functional mRNAs, let’s consider how regulation of this process can contribute to gene control. Recall from Chapter 8 that higher eukaryotes have both simple and complex transcription units encoded in their DNA. The primary transcripts produced from the former contain one poly(A) site and exhibit only one pattern of RNA splicing, even if multiple introns are present; thus simple transcription units encode a single mRNA. In contrast, the primary transcripts produced from complex transcription units (which constitute about 95 percent of all human transcription units) can be processed in alternative ways to yield different mRNAs that encode distinct proteins (see Figure 8-3).