Mandatory DNA sampling of people convicted of felonies to build the database has become pretty routine.  In 42 U.S.C. §14135a(a)(1)(A), Congress authorized collection "from individuals who are arrested, facing charges, or convicted or from non-United States persons who are detained under the authority of the United States."  Well, that's pretty broad.

Does that violate the Fourth Amendment?  USCA3 en banc said no 8-6.  Decision here.  Here are posts at How Appealing and Volokh Conspiracy.