  Another problem with this common definition is that it is based on an overly simplistic account of DNA expression. According to this simple account, a gene is a sequence of nucleotides in DNA that is transcribed into a sequence of nucleotides making up a messenger RNA molecule that is in turn translated into sequence of amino acids that forms a polypeptide. (Biologists talk as if genes “produce the polypeptide molecules” or  “provide the information for the polypeptide”.) The real situation of DNA expression, however, is often far more complex. For example, in plants and animals, many mRNA molecules are processed before they are translated into polypeptides. In these cases, portions of the RNA molecule, called introns, are snipped out and the remaining segments, called exons, are spliced together before the RNA molecule leaves the cellular nucleus. Sometimes biologists call the entire DNA region, that is the region that corresponds to both introns and exons, the gene.  Other times, they call only the portions of the DNA segment corresponding to the exons the gene. (This means that some DNA segments that geneticists call genes are not continuous segments of DNA; they are collections of discontinuous exons. Geneticists call these split genes.)  Further complications arise because the splicing of exons in some cases is executed differentially in different tissue types and at different developmental stages. (This means that there are overlapping genes.) The problem with the common definition that genes are DNA segments that “code for polypeptides” is that the notion of “coding for a polypeptide” is ambiguous when it comes to actual complications of DNA expression. Gene skeptics argue that it is hopelessly ambiguous (Burian 1986, Fogle 1990 and 2000, Kitcher 1992, and Portin 1993).