text: Despite the widespread classification of genes as either structural or regulatory, these categories are not an absolute division. Recent genetic discoveries call into question the distinction between regulatory and structural genes.
  The distinction between regulatory and structural genes can be attributed to the original 1959 work on Lac operon protein expression. In this instance, a single regulatory protein was detected that affected the transcription of the other proteins now known to compose the Lac operon. From this point forward, the two types of coding sequences were separated.
  However, increasing discoveries of gene regulation suggest greater complexity. Structural gene expression is regulated by numerous factors including epigenetics (e.g. methylation), RNAi, and more. Regulatory and structural genes can be epigenetically regulated identically, so not all regulation is coded for by “regulatory genes”.
  There are also examples of proteins that do not decidedly fit either category, such as chaperone proteins. These proteins aid in the folding of other proteins, a seemingly regulatory role. Yet these same proteins also aid in the movement of their chaperoned proteins across membranes, and have now been implicated in immune responses (see Hsp60)  and in the apoptotic pathway (see Hsp70).
  More recently, microRNAs were found to be produced from the internal transcribed spacers of rRNA genes. Thus an internal component of a structural gene is, in fact, regulatory. Binding sites for microRNAs were also detected within coding sequences of genes. Typically interfering RNAs target the 3’UTR, but inclusion of binding sites within the sequence of the protein itself allows the transcripts of these proteins to effectively regulate the microRNAs within the cell. This interaction was demonstrated to have an effect on expression, and thus again a structural gene contains a regulatory component.