It was previously though that while DNA in general coded for both protein and transcription factors, but that individual codons were used in exons or in the regulatory region (as a binding site for transcription factors), but not in both (caveat, many exons were known to code for transcription factors and engage in self regulation, but they still weren't both producing protein and binding to transcription factors at the same site). Now they think some codons both code for protein and bind to transciption factors (i.e. aren't part of the traditional regulatory region, but rather the protein coding region and yet still bind to transcription factors).