  Biological textbooks contain definitions of gene and it is instructive to consider one in order to show that the conceptual situation is indeed unsettling. The most prevalent contemporary definition is that a gene is the fundamental unit that codes for a polypeptide. One problem with this definition is that it excludes many segments that are typically referred to as genes. Some DNA segments code for functional RNA molecules that are never translated into polypeptides. Such RNA molecules include transfer RNA, ribosomal RNA, and RNA molecules that play regulatory and catalytic roles. Hence, this definition is too narrow.