Ab Initio gene prediction is an intrinsic method based on gene content and signal detection. Because of the inherent expense and difficulty in obtaining extrinsic evidence for many genes, it is also necessary to resort to "ab initio" gene finding, in which the genomic DNA sequence alone is systematically searched for certain tell-tale signs of protein-coding genes. These signs can be broadly categorized as either "signals", specific sequences that indicate the presence of a gene nearby, or "content", statistical properties of the protein-coding sequence itself. "Ab initio" gene finding might be more accurately characterized as gene "prediction", since extrinsic evidence is generally required to conclusively establish that a putative gene is functional.