A defendant in a criminal case has no obligation to prove his innocence. Obviously, if we have evidence that tends to prove innocence, we present it as a part of our criminal defense. We also do everything we can, within the bounds of the law and the Code of Professional Responsibility, to convince the trier of fact (the judge or the jury, as the case may be) that there is a "reasonable doubt" that the defendant is guilty. This is true whether you've been arrested on charges of drunk driving, DWI/DUI or Murder. Examinations of DNA evidence and forensic analyses often point to the defendant's guilt. In those cases, the defense may challenge the legality of the search which produced the evidence and sometimes, the validity of the prosecution's DNA testing procedure.