10/16/2008 05:12 am ET Updated May 25, 2011

What's the Police Union Thinking?

The union that represents Chicago police is advising its members involved in crime scene investigations to refuse to provide samples of their DNA for forensic analysis.

Providing the samples is eminently sensible for the simple reason that unidentified DNA at a crime scene can be counterproductive in bringing a perpetrator to justice. If nothing else, it gives a defense lawyer an opportunity to suggest that someone other than his client was at the scene and could have committed the crime.

So what's the objection?

"Our concerns," explains the Fraternal Order of Police web site, "are many, including but not limited to, whether or not [police officers'] DNA samples will be put into CODIS?"

CODIS (Combined DNA Index System) is a national database created and funded by the FBI. Its purpose is to identify criminals, primarily rapists and murderers, who otherwise would have escaped detection. It's served that purpose well, bringing thousands of violent criminals to justice.

The big downside to having your DNA in CODIS is that you'll likely to be nailed if you rape or kill someone.

What possible objection could a police union have to that?