It's a critical tool for investigators solving difficult crimes, but when is it legal for law enforcement to take a suspect's DNA?
That's the question being taken up today by the US Supreme Court after the 2009 conviction of a Maryland man for rape and murder was overturned on constitutional grounds.
The Maryland DNA Collection Act is a law that allows officials to take the DNA from those who have been arrested -- but not convicted of a serious crime.
Now, the question is -- is it constitutional?
The Supreme Court is expected to figure that out today as the case of Alonzo King takes center stage.
King was arrested in 2009 on assault charges. Police took his DNA. The sample matched DNA collected from the victim and then police linked him to a 2003 rape.
King's lawyers argue the first draw of DNA from kin violated his rights. Despite that, the lower court found King guilty of first degree rape and sentenced him to life in prison.
Last April, the Maryland Court of Appeals ruled against a key provision of the law.
While all states require DNA from individuals convicted of a felony, the federal government and 28 states also require DNA collection and analysis from at lease some arrestees.
For more on the debate over DNA in Maryland, click here.