The Fifth Amendment to the Bill of Rights. At least as long as a court witness has reasonable cause to apprehend danger from his or her answers if questioned at trial, the witness is permitted to assert the Fifth Amendment right to remain silent.  (From the public domain).

Thanks to a fellow criminal defense e-mail listserv member for posting a link to Ohio v. Reiner,  532 U.S. 17 (1991).  

Reiner confirms that the Fifth Amendment right to remain silent in court may even be exercised by people maintaining their innocence. So long as a court witness has reasonable cause to apprehend danger from his or her answers if questioned at trial, the witness is permitted to assert the Fifth Amendment right to remain silent. Ohio v. Reiner, 532 U.S. at 21.

Jon Katz.